The amino acid sequence of a lysozyme, (B-enzyme), from Bacillus subtilis YT-25 wasdetermined by conventional methods. B-Enzyme comprised 117 amino acid residues and had a heterogeneous sequence in the amino-terminal region. The amino acid sequenceof B-enzyme was different from those of all other lysozymes the sequences of which are known. However, the partial amino acid sequence of Ser (74) to Ser (97) of B-enzyme was homologous with that of the active-site region of hen egg-white lysozyme (Ser (36) to Ser (60)), which includes one of the catalytic amino acids, Asp (52). It is interesting that B-enzyme has an amino acid sequence homologous with that of the gagprotein p25 of the AIDS virus ARV-2.