We isolated three human testis-specific genes by a differential mRNA display method. The cDNAs contained open reading frames of 1620, 453, and 333 nucleotides, encoding 540, 151, and 111 amino acids, respectively. The first of these genes, designated TSA303, encodes a novel protein homologous to TCP20, one of the subunits of the human TRiC chaperonin complex that can bind newly synthesized or unstable folding intermediates of polypeptides and assist substrate proteins in folding, assembly, and transport. The second, TSA806, encodes a novel protein containing 3.3 contiguous repeats of the cdc10/swi6 (ankyrin) motif that was originally found in products of cell cycle control genes of yeast and cell fate determination genes in Drosophila and Caenorhabditis elegans. The third gene, TSA903, encodes a protein homologous to the C-terminal region of murine uridine monophosphate kinase. Northern blot analysis confirmed that in 16 human adult tissues examined, each of these genes was expressed specifically in the testis. From the results of cDNA screening of nearly 1 million plaques, the abundance of each transcript in a preparation of total mRNA was estimated as 0.0004% (TSA303), 0.0006% (TSA806), and 0.0002% (TSA903). Our results imply that the differential display method is a powerful tool for isolation of tissue-specific genes even if they are expressed at a level as low as 1 in several hundred thousand to a million molecules of total mRNA.