The production of extracellular proteinase by MY1049, a low-virulence mutant of Candida albicans, was compared with that of its wild-type parent, MY1044. Both strains were grown in a medium containing bovine serum albumin as a nitrogen source to induce proteinase production. Under these conditions, the proteinase activity per cell in the MY1049 cultures was the same as or higher than that of MY1044 cultures. However, MY1049 grew much more slowly than MY1044, and the total proteinase activity of the MY1049 culture remained well below that of the MY1044 culture. In a minimal medium with ammonium sulfate as the nitrogen source, MY1049 grew as rapidly as did MY1044. No significant differences were observed in the effects of inhibitors produced by MY1049 and MY1044. Our previous work indicated that MY1049 was able to grow and produce abundant mycelium in the renal calices of infected mice but that the strain was unable to invasively colonize the renal tissue. The decreased ability of MY1049 to grow in a protein-rich environment, despite its ability to produce extracellular proteinase, may enable the host to contain the mutant strain before the fungal cells invade the tissue.