The presence of leucocytes and their subpopulations was studied in the ejaculate from 69 men with an infertile marriage and 12 fertile men. Monoclonal antibodies specific for human leucocytes were used in an immunoperoxidase technique. In addition to the standard sperm analysis, each specimen was also tested for sperm antibodies, aerobic and anaerobic microorganisms, Ureaplasma urealyticum and Chlamydia trachomatis. Leucocytes were found in large numbers in the fertile men compared with the patients. Lymphocytes were found in 20% of the patients. Micro-organisms were cultured from a similar proportion of both groups. We found no correlation between leucocyte counts, sperm density and motility, sperm antibodies and growth of micro-organisms. Our results cast doubt on the conventional criteria of subclinical genital tract infection, namely positive culture and excess leucocyte counts.