We have studied the virulence for mice of Candida albicans strains with different proteinase activity in the culture. The mortality rates for mice infected with the type Ia strains, which secrete proteinase whose activity porsisted for a week in vitro, were higher than those infected with the type Ib strains, which secrete proteinase whose activity declined at 2 or 3 days in vitro and the type II proteinase-deficient strains. This was substantiated by the number of colony-forming units (CFU) recovered from kidneys of mice infected with C. albicans. In the kidney tissues of mice infected with the type Ia strains, extensive invasion by fungal cells and the secretion of proteinase were histologically demonstrated, while in those infected with the type Ib and II strains fungal cells were rarely found. However, the mice infected with the type Ib strain NUM 978 were an exception; the recovery of CFU from the kidney was high, but the animals survived longer. Histologically, Candida cells were not colonized but interspersed in the tissue. Type II strain NUM 584 was found to be moderately virulent when infected at a high dose. These observations indicate that the proteinase plays a role in type Ia strains but that other factors are involved in the type Ib or II strains for the establishment of pathogenicity of C. albicans.