The present invention relates to cystatin T (also known as zcys3) polypeptides and polynucleotides encoding the same. Cystatin T polypeptide is testis specific and homologous to cystatin-related epididymal specific gene (CRES) and type 2 cystatins. The polypeptides would be useful for modulating spermatogenesis and may be used to study or modulate that function in in vitro or in vivo systems. The present invention also includes antibodies to the cystatin T polypeptides.