The activity of bacitracin, streptomycin, penicillin and Na caprylate in varying dilutions were tested in in vitro studies against serial dilutions of organisms commonly isolated from root canals. The organisms used were: alpha Streptococcus, beta Streptococcus S. fecalis, Staphylococcus aureus S. albus Escherichia coli and Candida albicans. Each antibiotic tested was effective against a specific organism or group, although ineffective, even in high concns., against the remainder of organisms tested.