The H-Zb-negative B78H I clone (derived from B 16 melanoma) was transfected with the H-2Kb gene; 4 cell clones expressing membrane H-2Kb antigens and 2 control clones (transfected with pSV,neo alone) were used for studies of metastatic ability, immunogenicity, N K sensitivity and homotypic adhesion. The experimental metastatic capacity of H-2Kb transfectants in syngeneic mice was greatly diminished in comparison with control and parent cells. Both immunemediated and intrinsic properties of transfectants correlated with their lower metastatic ability. A cell-mediated cytotoxic response was induced by repeated in vivo immunizations of syngeneic mice followed by in vitro restimulation of effectors when transfectants (but not controls) were used as imrnunizers and as targets. Moreover, homotypic adhesion of H-2Kb transfectants was significantly lower than that of controls. Sensitivity to NK cells of transfectants was not decreased in comparison to ti-2-negative controls. It is known that in vitro treatment with IFN-y of H-2-positive B16 melanoma cells induces a simultaneous increase in H-2 expression and in experimental metastasis; treatment of H-2Kb transfectants with IFN-y induced a higher Kb expression, but no increase in metastatic ability, thus suggesting that the IFN-sensitive component that mediates enhancement of metastasis is not H-2Kb. 20