The CWLA lytic amidase of Bacillus subtilis 168 was purified and antisera raised against the purified protein. No expression of cwlA could be demonstrated under any conditions by the use of the antisera and cwlA::lacZ fusion analysis. Two lytic enzymes of apparent molecular masses 34 and 30 kDa (as measured by renaturing SDS-PAGE) were found to be mitomycin C-inducible, the larger of which corresponds to a protein immunologically related to CWLA. Both of these inducible lysins were found to be encoded by prophage PBSX. Prophage SP beta was shown by renaturing SDS-PAGE to produce a 43 kDa lytic enzyme unrelated immunologically to CWLA. The smaller of the two PBSX enzymes was purified and found to be an N-acetylmuramyl-L-alanine amidase of 32 kDa (as measured by SDS-PAGE and Coomassie blue staining) which cross-reacts only weakly with the anti-CWLA sera. The potential origin of cwlA and its possible relationship to the other phage lytic enzymes are discussed.