Pregnant mice which in theory differ only in the region of the major histocompatibility complex (MHC) on chromosome 17 (C57BL/10, the inbred partner [host strain], and B10.D2, B10.BR, B10.A, B10.A[2R], B10.A[5R], B10.A[15R] and B10.A[18R]) were sacrificed on the 11th and 18th days of gestation, and the fetuses were sexed and weighed. Fetuses from reciprocal crosses between B10.A and B10.BR, B10.D2 and C57BL/10 were weighed and sexed on the 18th day of gestation. It was found that (i) fetal weights were not significantly different among the strains examined on day 11 (B10.BR, B10.A[15R] and B10.A[18R]), (ii) B10.BR fetuses of both sexes weighed significantly less than fetuses from the other strains on day 18, (iii) B10.D2 18-day-old male but not female fetuses were heavier than the males from the other strains (this difference was not present when corrections for litter size were made), (iv) the fetuses from the B10.A x B10.BR cross were the smallest, those from the B10.D2 x B10.A cross the largest, and those from the B10.A x C57BL/10 crosses intermediate, and (v) maternal effects were noted in the B10.A x B10.BR and B10.A x B10.D2 but not the B10.A x C57BL/10 crosses. The results suggest that there are two or more MHC associated loci that influence growth rate in late gestation. Among the candidate genes are Ped and Igfr II.