1. The gestational age of donor hematopoietic tissue appeared to have no influence on the 30-day mortality following homologous transplants in irradiated mice.
2. Recipients of second trimester homologous donor tissue had a late (90-day) mortality that was 16 per cent lower than that observed in animals receiving tissue from third trimester and neonatal donors, but statistical analysis showed a low level of significance.
3. Irradiation alone appeared to cause a late mortality in non-injected irradiated animals and in isologously transplanted irradiated animals.