To evaluate antidental caries activity of polylysine cell growth and acid production of Streptococcus mutans and lactobacillus acidophilus were microbiologically monitored in anaerobic broth system containing various concentration of polylysine. The pH and heat stability of polylysine having antimicrobial activity were also examined. Two tested microbes were fairly well grown in broth containing polylysine 0.1mg/ml however inhibited at 1 and 2mg/ml of polylysine concentration. Especially lag times of Strep-tococcus mutans and Lactobacillus acidophilus were prolonged to about hour at 1.0 and 2.0 mg/ml of poly-lysine. acid production of Streptococcus mutans and lactobacillus acidophilus was also decreased by poly-lysine. Antimicrobial activity of polylysine was not affected by the change of pH and the heat treat-ment.