The relative fertilizing potential of frozen-thawed semen from four black and four white boars was determined following heterospermic insemination. A heterospermic index (HI) was computed for each of the 16 pairs of black and white boars, as the percentage of offspring sired by the black boar minus the percentage of offspring sired by the white boar. Correlation coefficients were computed between the HI and the differences (black boar minus white boar) in several in vitro tests of seminal quality before and after cryopreservation of the semen. For the in vitro tests before cryopreservation, the HI were negatively correlated (-0.57, P 0.05) with the differences in spermatozoan motility after cooling to 5(DEGREES)C. The HI were not correlated (P > 0.05) with the differences in the following in vitro tests after freezing and thawing: spermatozoa with normal apical ridges, damaged apical ridges, and loose acrosomal caps, extracellular and maximum releasable glutamic oxaloacetic transaminase (GOT), and the number of penetrations per zona-free hamster oocyte. The HI were correlated (P < 0.05) with the differences in the following in vitro tests after freezing and thawing: spermatozoan motility (0.50), spermatozoa with either normal or damaged apical ridges (0.31), spermatozoa with missing apical ridges (-0.51), spermatozoa filtered through sephadex columns (0.32), spermatozoa with acrosin-activity (0.38), percentage of maximum releasable GOT (0.54), spermatozoan intracellular GOT (-0.57), spermatozoa bound per zona-free hamster oocyte (0.64) and, percentage of zona-free hamster oocytes penetrated (0.75). The relatively low correlations between the HI and the differences in most in vitro tests indicated that the fertilizing potential of the semen could not be predicted with reasonable accuracy with any simple in vitro test. In contrast, the multiple regression correlation coefficient between the HI and the differences in four parameters from three in vitro tests (motility, spermatozoa with either normal or damaged apical ridges, number of spermatozoa bound per oocyte, and percentage of oocytes penetrated) was 0.94.