In our initial studies on the question of histocompatibility requirements in T-B-cell interactions, we found that no cooperation occurred with mixtures of T and B cells from BALB/c (H-2(d)) and A/J (H-2(a)) donors, respectively (1). These particular strains are identical for genes in the S and D regions of the H-2 complex but possess major differences at the K-end. Many differences are known to exist in the I region as well. Thus, these early data indicated that gene identities only at the D-end are insufficient to permit optimal cooperative interactions to occur under these conditions.