Antisera to the penicillinases of Bacillus subtilis strains 749 and 6346 were prepared, and the neutralization of enzymic activity by homologous and heterologous antisera was studied. Antisera to 749 penicillinase had, as expected, greater neutralizing power against 749 penicillinase than against 6346 penicillinase. A 6346 antiserum, however, neutralized the heterologous enzyme more effectively than the homologous enzyme, and was also more effective in precipitating heterologous than homologous enzyme activity. The penicillinases of B. subtilis, unlike those of B. cereus, could not be adsorbed on glass, but could be partially purified by calcium phosphate gel-adsorption. There was no cross-reaction, as measured by enzyme neutralization, between the penicillinases of B. subtilis and that of B. cereus 569. Materials other than penicillinase in the enzyme preparations used did not interfere with the neutralization reactions between B. subtilis penicillinases and their antisera. The basal penicillinase produced by B. subtilis 749 had the same neutralization curve, with a homologous antiserum, as did the induced enzyme.