Male mice carrying one complete t haplotype transmit it to virtually all offspring. This implies that either the sperm carrying the t haplotype have an advantage in fertilization, or that the sperm not carrying a t haplotype are dysfunctional. To distinguish between these possibilities, epididymal sperm from tw32/ + and congenic + / + males were artificially inseminated into the uterus of T / + females, and the transmission ratio determined from the phenotype of the offspring. After artificial insemination (AI) of sperm from tw32/ + males, the mean ratio was 0·95 (3 experiments, 5 litters), demonstrating that the transmission ratio remains high after AI into the uterus. After AI of a mixture of equal numbers of motile sperm from tw32/ + and congenic + / + males, the mean ratio was 0·22 (6 experiments, 8 litters). These results suggest that sperm carrying a t haplotype from t/ + males are not superior to sperm from + / + males, and therefore imply that sperm not carrying a t haplotype from t/ + males are dysfunctional.