Extracellular bacteriolytic activity was obtained when Gliomastix murorum var. felina was grown on heat-killed Bacillus subtilis cells (2.5 mg ml⁻¹) suspended in a defined salts solution, buffered at pH5 or at pH7. Maximal yields were measured when shake flask cultures were incubated in light at 30°C for 10 days. Lytic supernatants were designated E₁ and E₃ (fungus grown at pH5, supernatant optimally active at pH3.6 and pH8.0 respectively) and E₂ (fungus grown at pH7.0, supernatant optimally active at pH7.5). E₁ was investigated further and found to be inducible and to be repressed by glucose addition. E₁ was purified by ammonium sulphate precipitation and gel filtration on a Sephadex G-75 column. The pH optimum was 3.4 (ionic strength 0.051 and the molecular weight was estimated by gel filtration as 17000. The mode of action of the bacteriolytic enzyme was that of a β-N-acetylmuramidase. Specific activity was increased 7-fold (from 61.4 units mg⁻¹ protein to 44B units mg⁻¹ protein).