The tissue specificity of the intra-acrosomal protein SP-10 was examined by Northern blot and polymerase chain reaction (PCR) analysis. Messenger RNA from 36 tissues in the female baboon (Papio papio) was isolated, separated on agarose gels, transferred to nylon, and probed with either SP-10, beta-actin, or cyclophilin cDNA. Northern blots, which were processed at both low and high stringency, showed SP-10 to be expressed exclusively in the testis. The mRNA from each tissue was also reverse transcribed, and both SP-10 and beta-actin were amplified by PCR from the resulting cDNA. Ethidium bromide-stained agarose gels of the SP-10 PCR products showed three clear bands from the testis but no co-migrating bands from the other tissues. Southern blots of the PCR products showed that only the three bands in the testis were related to SP-10. The data demonstrate that the SP-10 gene products are testis specific, a characteristic essential for a contraceptive vaccine candidate molecule.