Female genotype effects on the transmission of t-haplotypes from heterozygous males to offspring has been established. The CBA females transmitted T-gene to 29 per cent, C57BL/6 females to 47 per cent, and F1 hybrid females to 39 per cent of their offspring. The differences in the transmission rate are supposed to depend on female genotype and to be determined by selective ageing of T-carrying sperms in female genital tract when compared to t6-carrying sperms, because the interval between copulation and syngamy in C57BL/6 females is shorter than in CBA females. These data indicate the phenotypical differences between T- and t6-carrying sperms.