ABO blood group substances were determined in either the seminal plasma (male patients) or saliva of 100 infertile couples and 100 control subjects. Comprehensive infertility evaluation was performed in all patients. Results were analysed in groups of couples with respect to their ABO blood group and compared with fertile control subjects. Over 70% of fertile marriages were compatible. Comparison of infertile and fertile couples for either ABO compatibility or incompatibility did not reveal any statistically significant difference (P greater than 0.05). This suggests some measure of low zone tolerance to ABO antigens on spermatozoa, for only a small minority of sperm introduced during coitus gains access to the oviduct and ultimately to the peritoneal cavity.