SUMMARY In fowl, erythrocyte isoantigens determined by the major blood-group histocompatibility locus (B) behave as carriers for hapten-type isoantigens. Results of the present study show that the “carrier” property of B isoantigens extends to cellular immunity of the allograft type. Among recipients of a skin graft possessing only minor histocompatibility antigens, accelerated rejection occurred if a prior skin graft possessing both foreign B and the minor antigens had been rejected. Accelerated rejection did not occur if the first graft was B-compatible. When B-incompatible spleen cells were used, instead of a first-set skin graft, survival of subsequent skin grafts possessing minor histocompatibility antigens was prolonged significantly. Apparently, these opposite effects are attributable to the augmentation of cellular immunity in the first instance, while, in the second instance, augmented humoral immunity to the minor antigens resulted in immunological enhancement.