A basic 54-kDa protein (pI approximately 8.8) that cross-reacts with anti-caltrin antisera has been detected and isolated by gel filtration and cation exchange chromatography from seminal vesicle content of the rat. The soluble protein spontaneously precipitated in NaHCO3-buffered solution at pH 7.8, but it was kept soluble in imidazole buffer containing EDTA and dithiothreitol at pH 7.0. In addition to the main band of 54 kDa, two faint immunoreactive fractions with molecular weights around 45,000 and 14,000 were also revealed by Western blotting. The presence of the rat caltrin sequence within the primary structure of the 54-kDa molecule has been investigated by sequencing the peptides generated by trypsin digestion. The sequence of the first 46 amino acid residues of rat caltrin has been found in one of the fragments produced by enzymatic cleavage. However, the exact location of the caltrin sequence in the whole 54-kDa protein has not been determined. The purified 54-kDa protein did not inhibit Ca2+ uptake by epididymal spermatozoa. Results indicated that this molecule represents an inactive precursor of caltrin and is enzymatically processed in the lumen of the seminal vesicle to the small and active calcium transport inhibitor protein. The immunoreactive proteins with intermediate molecular weights (45,000 and 14,000) could represent partially degraded products of the precursor. The lack of inhibitory activity of the precursor may be related to the molecule's having a size and conformation that would make it unable to interact with caltrin receptors on the sperm surface.