Summary: Two mutants of deficient in autolysin activity produced a protein that showed immunological identity with the -acetyl-muramyl--alanyl-amidase present in the wild-type strain, when tested with antiserum obtained against this enzyme. The protein was produced by the mutant cultures grown either at 37 °C or at 30 °C, although only the cell extracts obtained at 30 °C showed significant cell wall hydrolysing activity. In contrast to the lysis resistance of these bacteria grown at 37 °C, mutant cultures grown at 30 °C exhibited significant degrees of autolysis when treated with detergent or cell wall inhibitors. Extracts of the mutant cultures contained a cell wall hydrolysing activity that was rapidly inactivated during incubation at 37 °C.