Working with the recently available recombinant haplotypes of the rat major histocompatibility complex (MHC)—RT1, we investigated the effect of various types of blood transfusion (BT) on allograft prolongation, including blood identical for the whole RT1 haplotype with that of the donor or for only a part of it. One or two milliliters of donor blood significantly prolonged graft survival in the (LEWXBN) F 1$$ LEW or the LEW. 1W $$ LEW. 1A combination. The optimal regimen consisted of two BTs given 15 and 7 days prior to grafting; BTs given at day-30 were ineffective. A BT given on the day of the operation was effective, but sequential BTs after grafting did not further increase graft survival. In the (LEWxBN) F 1$$ LEW combination, blood from congenic LEW. 1N rats significantly prolonged graft survival, but third-party BTs were ineffective or had only a borderline effect when transfused (1 ml, 8 times) within the three months before transplantation. This showed the major role of the RT1 system as well as the specificity of the model.