Abstract.Cytotoxicity of pooled extracts from bacterial plaque was demonstrated by inoculating organ cultures of fetal rat masticatory mucosa. Advanced cytotoxic reactions were characterized by pyknosis, karyorrehexis, hyperkeratosis and possible acantholysis. Histochemical microscopy indicated possible alteration in Krebs cycle function and an increased anaerobic metabolism. Quantitative histochemistry demonstrated a statistically significant increase in total lactic dehydrogenase, glucose‐6‐phosphalc dehydrogenase and acid phosphatase in test explants when compared to controls. Test explants also exhibited a significant decrease in malic and isocitric dehydrogenase activit