A-Protein was isolated from a purified male-specific filamentous phage fd particle. A-Protein has a molecular weight of approximately 60,000 daltons, as estimated by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. The amino-terminal residue was glycine as determined by the dansylation technique. Amino acid analysis showed that histidine, arginine, and cysteine, which are not contained in B-protein, are present in A-protein.