The molecular mass of proteinase K was determined by gel electrophoresis in the presence of sodium dodecyl sulfate and by active site labelling with diisopropyl fluorophosphate. Both methods indicate molecular masses in the range of 27 000-29 000 Da. These values differ significantly from that of 18 500 formerly determined by gel filtration (Ebeling et al. (1974) Eur. J. Biochem. 47, 91-97). Proteinase K was inactivated with [3H]diisopropyl fluorophosphate. Afterwards the labelled protein was reduced, S-carboxymethylated and digested with cyanogen bromide. The chain lengths of the isolated CNBr-fragments are indicative of a molecular mass of proteinase K of at least 28 000 Da. Two CNBr-fragments were sequenced. The radioactively labelled fragment contains 69 residues and the sequence around the labelled residues was found to be -Ile-Ser-Gly-Thr-SER-Met-Ala-Thr-Pro-. This sequence is typical for that around the active site residue of the subtilisins. From the determined sequences it is concluded that the fungal proteinase K is phylogenetically related to the bacterial subtilisins.