The membrane penicillinase of Bacillus licheniformis 749/C is a phospholipoprotein carrying extra residues of asparagine or aspartate, serine, glutamine or glutamate and glycine not present in the exoenzyme (Yamamoto, S., and Lampen, J.O. (1976) J. Biol. Chem. 251, 4095-4101). Cleavage of the membrane enzyme with trypsin yielded a phospholipipopeptide and a hydrophilic penicillinase differing from exopenicillinase only by the absence of the NH2-terminal lysine residue. Phosphatidylserine was isolated from a pronase digest of the phospholipopeptide. The partial sequence of the phospholipopeptide is: phosphatidylserine-(Ser3, Glx5, Asx7, Gly5)-Asp-Gin-Ser-Lys-COOH with the lysine being the NH2-terminal residue of the usual exoenzyme. The fatty acids present in the membrane enzyme and in the phospholipopeptide had essentially the same composition (predominantly n-16:0, ante iso-17:0, n-18:0, and n-18:1). These acids were also found in the total membrane lipids, although in very different proportions; thus, the phosphatidic acid residue of the phosphatidylserine is probably formed by the usual synthetic pathway for membrane phospholipids, but some special feature of the process affects the nature of the component fatty acids.