Zoocin A is a lysostaphin-like streptococcolytic enzyme produced by Streptococcus equi subsp. zooepidemicus 4881 that specifically targets the cell walls of some closely related species. On the basis of sequence homology it was suggested that zoocin A was a domainstructured enzyme with the N-terminal domain responsible for catalysis (CAT) and the C-terminal domain for target recognition (SBD). Polypeptides corresponding to zoocin A (rZooA) and each of the putative domains (rCAT and rSBD) were prepared by use of recombinant technology. The biological activities of each was compared by use of a dye-release assay and a cell-binding assay. Cell wall hydrolysis was shown to be a function of CAT and target recognition a function of the SBD. Expression of the zoocin A immunity factor gene produced cell walls resistant to hydrolysis by either rZooA or its component domains, and with reduced capacity to bind rZooA and rSBD.