To investigate the role played by mutations of LDHC gene in male infertility, 100 male patients with unexplained infertility were screened by activity staining of LDH-C₄ on sperm smears and those with lowered or no LDH-C₄ activity in their sperms were subject to DNA analysis. The encoding exons were PCR amplified and the PCR products were screened by DHPLC for mutations. The PCR products with abnormal elution peaks were subject to DNA sequencing. In 14 infertile male patients with lowered LDH-C4 activity in sperms, it was identified a heterozygous T > A mutation in base 115 of exon 5 of LDHC gene(GenBank accession number: GU479375), changing codon 178 (TTG, encoding leucine) to a stop codon (TAG). This nonsense mutation (L178X) was predicted to result in a severely truncated protein. The mutation and the heterozygosity were further confirmed by T-cloning. To the best of the knowledge, this is the first mutation found in human LDHC gene, substantiating that mutations in LDHC gene might cause male infertility.