Minocycline (MC) and Doxycycline (DC) are used worldwide as locally applied adjuncts in the treatment of periodontal diseases. As a group the tetracyclines are well known for their advantageous properties. There is however possible cytotoxicity towards cells in the area of application. This study determined the minimum cytocidal concentration of MC and DC on the growth and proliferation of Human Periodontal Ligament Fibroblasts (HPLF) in vitro. This was facilitated by growing cells (PDL1 and PDL2) in the presence of MC and DC in media in 96 tissue wells starting at a concentration of 1400 µg.ml-1 (100%). Serial dilutions of the MC and DC at 10% increments were investigated in order to detect significant HPLF cell growth inhibition. The significant LD50 was further determined at one percent increments in order to arrive at a specific percentage value. The results were read as LD50 values from growth concentration curves. The LD50 of MC on PDL1 and PDL2 after one hour exposure was 686 µg.ml-1 and 896 µg.ml-1 respectively while for DC it was 252 µg.ml-1 and 546 µg.ml-1. The LD50 of MC on PDL1 and PDL2 after 24 hour exposure was 196 µg.ml-1 and 266 µg.ml-1 respectively while for DC it was 252 µg.ml-1 for both. The LD50 of MC on PDL1 and PDL2 after 48 hour exposure was 252 µg.ml-1 and 182 µg.ml-1 respectively while for DC it was 154 µg.ml-1 and 168 µg.ml-1. Based upon the LD50 values this study found that DC is more cytotoxic than MC and linked to this, the two cell lines reacted slightly differently. The concentrations MC and DC tested in this study did however not influence growth of HPLF significantly. / Dissertation (MChD)--University of Pretoria, 2010. / Oral Pathology and Oral Biology / unrestricted