SUMMARYPassive enhancement of H‐2 incompatible skin allografts was studied in systems where the enhancing serum covered only a restricted number of Ia specificities involved in the incompatibility. Sera reacting with products of the I‐A region were effective at prolonging survival, probably to the same extent as a serum covering all the specificities of the incompatibility. Such sera also inhibited MLR in the same strain combinations. Interpretation of these and other data suggests that products of the I‐B, I‐E and I‐C regions play a subordinate role as target antigens for enhancement. It is concluded that either Ia antigens of different regions have different biological activities or enhancement is a function only of antibodies directed against antigens that stimula