Several body fluids and various substances were examined for their effects on the action of viridin B, a Streptococcus mitis bacteriocin. Heme has been shown previously to interfere with viridin B action. In the present study, peroxidase and catalase, but not superoxide dismutase, interfered with the bacteriocin. Sulfhydryl-containing compounds had no effect. Trypsin, protease, and amylase exhibited interfering capacities. Of various body fluids tested, serum, saliva, and leukocyte lysate interfered with viridin B. The activity in leukocyte lysate was probably due to myeloperoxidase. Salivary activity was likely due to both peroxidase and amylase. Interfering activity in serum was neither complement dependent nor antibody mediated and was probably due to a combination of enzymatic activities. Temporal studies with these agents suggested more than one mechanism of interference with viridin B.