A mouse t-complex-specific DNA probe was used to determine the ratio of t-carrying and (+)-carrying sperm in epididymal, vas deferens, and postejaculatory sperm cell populations from heterozygous (+/t) mice with transmission ratios of greater than 95%. No detectable degeneration of (+)-carrying sperm was observed. In this respect, mouse t haplotypes differ from Drosophila melanogaster SD chromosomes. High transmission of t haplotypes must be a consequence of differential transport and/or differential sperm function during the fertilization process itself.