We have previously demonstrated that transfection of BL6-8 melanoma cells with the H-2Kb gene increased their sensitivity to natural killer (NK)- and natural cytotoxic (NC) cell-mediated cytotoxicity, and resulted in alterations of various cell characteristics such as the loss of melanoma-associated antigen (MAA), and the appearance of Griffonia simplicifolia 1B4 (GS1B4)- and soybean agglutinin (SBA)-lectin-binding sites. In the present study the BL6-8 melanoma clone was transfected with the H-2Kb gene without cotransfection of the neomycin resistance (neor) gene, and transfected cells were isolated on the basis of their low sialylation and ability to bind to SBA-agarose beads. 38 out of 47 analyzed clones were morphologically distinguishable, expressed the H-2Kb gene, lost the MAA and gained SBA- and GS1B4-lectin-binding sites (H-2Kb+, MAA-, SBA+, GS1B4+), whereas only 5 clones were morphologically and phenotypically unchanged (H-2Kb-, MAA+, SBA-, GS1B4-). The remaining 4 clones were morphologically changed, lost MAA and gained SBA- and GS1B4-binding epitopes, but did not express H-2Kb antigen (H-2Kb-, MAA-, SBA+, GS1B4+). Analysis of the NK sensitivity of H-2Kb-gene-transfected clones revealed that H-2Kb+, MAA-, SBA+, GS1B4+ clones were highly sensitive to NK cell lysis, whereas H-2Kb-, MAA+, SBA-, GS1B4- clones were NK resistant. It is of particular note that the 4 clones that did not express H-2Kb antigen but had other phenotypic changes (H-2Kb-, MAA-, SBA+, GS1B4+) were also highly sensitive to NK cells. The NK sensitivity of H-2Kb-transfected cells was closely associated with their increased ability to compete with YAC-1 cells for effector cells in a cold-target inhibition assay. Thus, the data obtained indicate that H-2Kb molecules in BL6-8 melanoma cells are not required for interaction with the effector cells, and H-2Kb-induced alterations in membrane carbohydrates and/or expression of the retrovirus-associated MAA might be important for the increase NK sensitivity of BL6-8 melanoma cells.