Infectious diseases such as pneumonia still pose a major global health concern. Currently, the world is facing widespread emergence of acquired bacterial resistance to antibiotics which constitute one of the chief causes of infectious diseases. The accumulation of different antibiotic resistance mechanisms within the same strains has induced the appearance of the so called “superbugs”, or “multiple-drug resistant bacteria”. Due to antibiotic resistance, attention is currently being drawn towards biologically active components isolated from plant species commonly used as herbal medicine, as they may offer a new source of antibacterial, antifungal and antiviral activities. This is the basis of this study. In this study four medicinal plants namely, Cassia abbreviata, Geranium incanum, Pelargonium hortorum and Tecoma capensis were investigated for their antimicrobial potential. In vitro antimicrobial activity using agar disc diffusion method, agar dilution method and broth microdilution plate determination of minimum inhibitory concentration (MIC), were carried out against ATCC (American Type Culture Collection) strains and clinical isolates known to cause pneumonia. Aqueous, methanol and acetone extracts from the selected plants were thus tested against strains of Streptococcus pneumoniae, Haemophilus influenzae, Staphylococcus aureus, Klebsiella pneumoniae, Pseudomonas aeruginosa, Acinetobacter baumannii, Escherichia coli and Candida albicans. The plants exhibited pronounced antimicrobial activity and were more active against Gram-positive bacteria than Gram-negative bacteria. During agar disc diffusion method, the highest inhibition zone was demonstrated by the acetone extract of P. hortorum (IZ=22mm and AI=0.73) against the reference strain of S. pneumoniae (ATCC 49619). The range of zones of inhibition in diameter across strains of S. pneumoniae and H. influenzae was 7mm to 22mm with activity index range of 0.23 to 0.74. The lowest MIC produced by medicinal plants in the study during agar disc diffusion method against S. pneumoniae and H. influenzae strains, was 2.5mg/ml. In broth microdilution plate assay, the lowest MIC demonstrated by C. abbreviata, T. capensis and P. hortorum extracts on tested bacteria was 0.031mg/ml and that of G. incanum was 0.063mg/ml. Candida albicans strains were only inhibited at 20mg/ml by the study plants. The highest activity among the individual extracts was shown by P. hortorum methanol extract which inhibited 71% of the studied bacteria. T. capensis methanol extract was the least and inhibited only 17% of the tested bacteria. The strains of Klebsiella pneumoniae showed the highest resistance to medicinal plants employed in this study. Traditional preparation of selected medicinal plants did not show any significant antimicrobial activity. Bioactive analysis of compounds on study plants was carried out using standard methods which revealed the presence of anthraquinones, flavonoids, phytosterol, saponins, tannins and triterpenoids. Comparison of the inhibitory effect of the plant extracts against some broad spectrum antibiotics revealed that the tested medicinal plants showed greater antimicrobial activity than standard antibiotics. However, there was no correlation between the antibiotic susceptibility patterns of the bacteria and the effects of the plants, signifying that plants probably function through different mechanisms. Bioautographic findings on thin-layer chromatography plate, exhibited clear zones of inhibition of bacterial growth with the Rf value range of 0.09 to 0.94. Anti-mutagenic activity was assayed by the Ames mutagenicity test in the plate-incorporation method using histidine mutants of S. typhimurium strains TA 100. The selected plant extracts at 2.5mg/ml and 5mg/ml did not induce mutagenesis in the absence of liver-metabolizing enzymes. The study results indicated that the selected plants are capable of inhibiting the growth of the studied pathogenic microorganisms to a varied degree. The leaves of G. incanum, P. hortorum, T. capensis as well as the stem bark of C. abbreviata could be novel sources of antimicrobial agents that might have broad spectrum activity. The anti-mutagenic properties of the studied medicinal plants may also provide additional health supplemental value to the other claimed therapeutic properties of the plants.