SUMMARYThirty‐one combinations of congenic and recombinant mouse strains were tested for heart and skin allograft survival times with and without ALS and the results compared with those obtained for MLR and CML. Although the genes for Lads, Ir and Ia specificities are all in the K end, on the average no greater strength of K end incompatibilities than D end were found. Some of the shortest survivals were D region only incompatible.We specifically wanted to find out if there were any special genes concerned with the control of antigenic strength within the H‐2 complex and if so, did they correlate with those already described for CML—the effector cell stimulating or ECS genes. We found no evidence that the ECS genes in themselves were concerned with allograft strength but instead found evidence for certain genes in the I region and others outside the I region which could be acting as allograft potentiating genes. In some combinations there was an association with the ECS genes and the allograft potentiating genes, and we concluded that in these combinations the two types were closely linked but separate.We wanted to know if there was a specific lymphocyte subpopulation concerned with antigenic strength that was sensitive to ALS; the results of our experiments were consistent with this hypothesis but did not pro