SUMMARYGraft‐versus‐host reaction, after injection of parental thymus cells into F1hybrid recipients, was measured by popliteal lymph node assay in combinations of congenic strains which involve incompatibilities at various regions of the H‐2 gene complex. The results indicate that incompatibility at genes in the I‐A, I‐B regions or their vicinity results in the strongest graft‐versus‐host reaction. Incompatibility at genes associated with the I‐E region also was effective, while no graft‐versus‐host reaction was seen in combinations with I‐C, S, and D r