Three antibacterial proteins were purified from the culture medium of NIH-Sape-4, an embryonic cell line of Sarcophaga peregrina (flesh fly). Sequencing studies showed that two of these proteins belong to the sarcotoxin I family, potent antibacterial proteins purified from the hemolymph of Sarcophaga larvae, whereas the other protein, named sapecin, is a new protein consisting of 40 amino acid residues including 6 cysteine residues. Unlike sarcotoxin I, sapecin preferentially represses the growth of various Gram-positive bacteria. The proteins of the sarcotoxin I family produced by this cell line were found to have carboxyl-terminal glycine, whereas sarcotoxin I in the hemolymph has amidated amino acids. This suggests that the embryonic cells lack an enzyme that cleaves off carboxyl-terminal glycine to form a new amidated carboxyl terminus.