The appearance of cytochrome P-450 and of cytochrome oxidase aa; were determined in the sophorose lipid producing yeast Candida (Torulopsis) apicola IMET 43747 grown on a mixture of glucose and n-hexadecane. Cytochrome P-450, detectable in both the logarithmic and the stationary growth phase was not repressed by glucose. At the end of the logarithmic growth phase the content of cytochrome P-450 was three- to fivefold increased, which was connected with initiation of sophorose lipid biosynthesis. After that it dropped to the basal level, which remained constant during sophorose lipid biosynthesis. Cytochrome P-450 from logarithmic cells was cross-reactive with an antibody derived against cytochrome P-450alk from C. tropicalis. With microsomal proteins of stationary cells no cross-reactivity was obtained. The microsomal hydroxylase system of stationary cells seem to be regulated by the carbohydrate used as carbon source.