Transplantation tolerance was induced in mice by inoculating newborn animals with semi-allogeneic haematopoietic cells. The mice rendered tolerant were treated within the first week of birth, or at the time of grafting (age 7-8 weeks), with recombinant interleukin-1 (rIL-1) or interleukin-2 (rIL-2). The effects of these treatments on tolerance induction were monitored in terms of skin allograft survival. Treatment of newborn mice with rIL-2 abolished tolerance induction in nearly all tested animals. When administered at the time of grafting, both rIL-1 and rIL-2 decreased the proportion of tolerant animals. However, these modulation effects of interleukins were only observed in strain combinations with genetic differences at the K end of H-2 or in the entire H-2 complex, in which it is difficult to establish permanent tolerance; no effects of interleukins on tolerance induction were found in a strain combination with a relatively weaker genetic barrier represented by incompatibility at the D region of the H-2 complex.