Microgard, a commercially-available pasteurized fermented milk, was found inhibitory to gram negative bacteria such as Pseudomonas putida, some well-known foodborne pathogens (e.g. Salmonella, Yersinia and Aeromonas) and some fungi as revealed by an agar incorporation assay method. Gram positive bacteria, however, were not inhibited. Rather, some bacteria in this group were stimulated by Microgard. The inhibitory activity of Microgard was optimum at pH 5.3 and below. Thermal stability and protease sensitivity were characteristic of Microgard. Addition of albumin or some emulsifying substances antagonized Microgard whereas treatment with proteases at an initial pH of 11.0 enhanced its inhibitory ability. Purified eluates of Microgard from a Sephadex Column (gel filtration) exhibited thermal stability and maximum UV absorbance. Furthermore, they gave positive results with the Biuret test, indicating the presence of protein type substance(s). Diacetyl, known to be inhibitory for psychotrophic bacteria, was present in Microgard as well as several organic acids. Filter-sterilized fermentation growth metabolites of Pediococcus spp., Propionibacterium spp., Bifidobacterium longum, Lactobacillus plantarum and a black-pigmented yeast were tested for antimicrobial activity as Microgard had been tested. Results revealed that some of these metabolites were inhibitory to Pseudomonas putida and Listeria monocytogenes, which represented food spoilage and pathogenic bacteria, respectively. / Graduation date: 1989