Mutants of the yeast Saccharomyces cerevisiae, devoid of proteinase yscD activity, were isolated by screening for the inability of mutagenized cells to hydrolyze Ac-Ala-Ala-Pro-Ala-beta-naphthylamide in situ. One of the selected mutants bears a thermolabile activity pointing to the gene called PRD1 as being the structural gene for proteinase yscD. All mutants isolated fell into one complementation group. The defect segregates 2:2 in meiotic tetrads indicating a single gene mutation, which was shown to be recessive. Diploids heterozygous for PRD1 display gene dosage. The absence of proteinase yscD did not affect mitotic growth under rich or poor growth conditions, neither mating nor ascopore formation. Also growth of mutant cells after a nutritional shift-down was not altered. Inactivation of enzymes tested which are subject to carbon-catabolite inactivation, a process proposed to be of proteolytic nature, is not affected by the absence of proteinase yscD. Protein degradation rates in growing cells, in cells under conditions of differentiation or heat shock, showed no obvious alteration in the absence of proteinase yscD activity. Also inactivation of alpha-factor pheromone was not affected by proteinase yscD absence. Normal growth of mutant cells on glycerol indicates that the enzyme is not involved in any vital event in mitochondrial biogenesis.