Principal cells of the mouse caput epididymidis synthesize and secrete a 24 kDa protein able to bind to the head of the spermatozoa. Sequencing of several clones selected from cDNA and genomic libraries, combined with the microsequencing of the NH2 terminus of the protein allowed to reconstitute the entire primary structure of the mature 24 kDa protein. It revealed 81% homology with a human plasma glutathione peroxidase and 61% homology with a mouse erythrocyte glutathione peroxidase. This enzyme, once secreted in the epididymal fluid, might protect sperm membrane lipids, particularly those of the acrosomal part, against peroxidation.