T(5;19)1Wh × T(9;19)163H F1 hybrids were used in four types of crosses to produce fetuses with trisomy 19. Trisomy was more frequent early in gestation (7.9–23.0 %) than after 16½ days (4.5–16.9 %) and was associated with many non-viable implantations. Male hybrids produced significantly more trisomics than female hybrids in crosses with NIH GP mice terminated early in gestation. In fetuses beyond 16 V2 days, isolated cleft palate was consistently associated with trisomy 19. It was found only in trisomics carrying one T1Wh and two T163H translocations, although trisomics with one T163H and two T1Wh or with single T163H and T1Wh translocations were also produced. These and other observations demonstrate the effects of genetic background upon frequency of trisomy, survival of conceptuses, and phenotype of trisomic fetuses. These effects, as well as trans-location-specific meiotic mechanisms, must be considered when mouse translocation trisomy is compared with that of h