Passive 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 legion 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 eider Ia antigens of different regions have different biological activities or enhancement is a function only of antibodies directed against antigens that stimulate in MLR. * This serum may also react with another, as yet unassigned, Ia specificity of the I-A or I-B region present in H-2f, H-2" and H-2b (J. R. Archer, personal communication).

H‐2 alloantisera contain Ia antibodies whose detection is obscured by the presence of serodominant K, D specificities. A method is described whereby Ia antisera can be prepared by absorbing conventional H‐2 sera with donor‐strain erythrocytes. The analysis of such absorbed antisera has led to the identification of the new specificity Ia. 16, and the redefinition of the specificities H‐2.34 and H‐2.46 as complex Ia specificities which map in the I‐A region of the H‐2 complex. A number of other so‐called H‐2 specificities also appear to have Ia‐like properties.