Growth of Micromonospora chalcea on a defined medium containing laminarin as the sole carbon source induced the production of an extracellular enzyme system capable of lysing cells of various yeast species. Production of the lytic enzyme system was repressed by glucose. Incubation of sensitive cells with the active component enzymes of the lytic system produced protoplasts in high yield. Analysis of the enzyme composition indicated that beta(1 --> 3) glucanase and protease were the most prominent hydrolytic activities present in the culture fluids. The system also displayed weak chitinase and beta(1 --> 6) glucanase activities whilst devoid of mannanase activity. Our observations suggest that the glucan supporting the cell wall framework of susceptible yeast cells is not directly accessible to the purified endo-beta(1 --> 3) glucanase and that external proteinaceous components prevent breakdown of this polymer in whole cells. We propose that protease acts in synergy with beta(1 --> 3) glucanase and that the primary action of the former on surface components allows subsequent solubilization of inner glucan leading to lysis.