Six semicongenic lines carrying different t haplotypes on the background of strain C57BL/10Sn (B10.t strains) and a (B10 ×T/t0) F1 hybrid were tested against one another in the mixed lymphocyte reaction (MLR) and cell-mediated lymphocytotoxicity (CML) assays. In every instance, the MLR results paralleled those of the CML typing: strain combinations giving a positive result in one assay gave a positive result in the second; combinations in which no response was observed in the MLR assay also failed to kill target cells specifically in the CML assay. Furthermore, the MLR and CML results were concordant with the results of the serological typing of these strains, as reported previously by us. The combined results suggest sharing of H-2 hyplotypes between B10.t12 and B10.t32, between B10.t6 and B10.tw1, and between B10.tw2 and (B10. ×T/t0) F1. These data support the conclusion, reached in our previous publication, that members of the same t-complementation group, with few exceptions, share H-2 haplotypes. © 1976 Springer-Verlag New York Inc.