As a 1st step in the study of the possible relationship between the T/t and H-2 complexes, the H-2 antigenic composition of the strains carrying factors t12, tw32, tw2, tw8, t1, t0, t6, tw1, tw71, tw73, tw12, tw5, tw75, and t38 was studied by using a battery of antisera containing antibodies against inbred-derived H-2 antigens. Five t strains (t12, t6, tw5, tw1, and tw2) were selected for the production of antisera against the H-2 complexes carried by t chromosomes. Spleen, lymph node and thymus cells from H-2b/t heterozygotes and tw2/tw2 homozygotes were injected into appropriate F1 hybrids between 2 inbred strains that carried the inbred-derived H-2 antigens of the donor. Four new H-2 antigens and 1 Ia antigen were uncovered and were assigned the symbols H-2.106 through H-2.109 and Ia.101, respectively. Three new H-2 haplotypes were also described, based upon the H-2 antigenic pattern of 3 t factors, t12, tw1, and tw5. These new haplotypes were given the symbols H-2t12, H-2tw1, and H-2tw5. When the t factors were grouped according to their H-2 haplotypes, their distribution, with certain exceptions, corresponded to the complementation groups. Thus, t chromosomes in the same complementation group carried similar, if not identical, H-2 haplotypes, despite the fact that these chromosomes were derived from widely separated geographic areas. Such an association between the t and H-2 complexes is most unusual in light of what is known of the polymorphism of H-2 haplotypes in wild mice populations. It suggests more than a casual relationship, at least at the population level, between the t and H-2 loci.