The effect of specific antisera on the activity of penicillinases from Bacillus licheniformis and Bacillus cereus depends on the strain of penicillinase used, the type of substrate undergoing hydrolysis and the proportion of antiserum to antigen present. Enzyme function may be either decreased or increased. The antisera appear to contain at least two types of antibody, present in different proportions, capable of acting as stimulators or inhibitors of enzyme function, according to the nature of the substrate. The observed effects seem best explained by supposing that combination with antibody involves conformational changes in the enzyme which profoundly affect the properties of the active site.