Spermatogenesis is a complex differentiation process which requires the coordinate synthesis of diverse stage-specific proteins. In attempting a large-scale identification and characterization of those proteins, we have made use of the recently described mRNA differential display method (Liang and Pardee, Science 257: 967-971, 1992). This method is based on the reverse transcription of mRNAs obtained from two different cell populations (pachytene spermatocytes and spermatids in the present study) followed by a PCR reaction and comparison of the individual cDNA populations in a polyacrylamide gel system. Up to the present we have been able to identify 268 cDNA bands. Most of them (77%) are common to both cell stages. From the differentially expressed bands (23%) an ample majority was spermatid-specific (74%). According to our present results we conclude that the mRNA differential display is a promising approach for investigations on stage-specific gene expression during a differentiation process like spermatogenesis.