Friesian cows were inseminated with semen mixtures containing equal numbers of spermatozoa from a Friesian and a Hereford bull. The five bulls of each breed gave twenty-five possible combinations. The paternity of calves was established by inspection of colour and conformation. Heterospermic indices were calculated to express the relative ability of sires to father offspring after mixed insemination. There were significant differences between the heterospermic indices of bulls, the maximum observed difference being twenty-one-fold. The indices were consistent over two series. The homospermic index was defined as the 16-week non-return rate after normal single first inseminations. The heterospermic index established differences between bulls more efficiently than the homospermic index; one estimate showed that the heterospermic method needed less than 1/170th the number of inseminations required by the homospermic method. The homospermic index was predictable from the heterospermic index, the regression coefficient having a significance level of 0·05 > P > 0·025. The initial spermatozoan concentration of a bull's semen (before dilution) was highly correlated with the heterospermic index. Measures of semen quality based on the morphology and staining affinity of spermatozoa predicted heterospermic and homospermic indices non-significantly but in the right direction.