Mouse seminal vesicles secrete four major protein components with estimated molecular masses of 95, 38, 17, and 16 kDa. Amino acid sequencing revealed that the 95‐kDa component represents a protein with an unknown structure, while the 38‐kDa component was identified as semenoclotin, the 17‐kDa component as seminal‐vesicle‐secreted protein IV, and the 16‐kDa component as seminal‐vesicle‐secreted protein V. Semenoclotin and the 95‐kDa component were readily cross‐linked by transglutaminase, suggesting that the two proteins are involved in the formation of the mouse copulatory plug. Treatment of mouse seminal vesicle fluid with human prostate‐specific antigen rapidly degraded semenoclotin, indicating a structural resemblance of this protein to human semenogelins, despite the vast difference in primary structure. As previously reported for other seminal‐vesicle‐secreted proteins, the semenoclotin transcripts are shown to be under androgen control.