Semen characteristics were studied in 96 men from an in-vitro fertilization and embryo transfer programme. Along with the routine semen analysis, the presence of sperm antibodies in seminal plasma was measured by an ELISA technique. Antibodies to spermatozoa (IgA and/or IgG) were present in 19 cases and 15 of these (78.95%) had abnormally high viscosity, often associated with a high percentage of particulate debris and an increased number of morphologically abnormal spermatozoa.