Cell surface antigens, the expression of which is highly enhanced along with the transformation of cells, were analyzed. W14 and W31, EJ-ras oncogene-induced transformants of a WKA rat fetus-derived fibroblast WFB, strongly expressed several transformation-associated antigens as defined by monoclonal antibodies 109, 061, and 081. These monoclonal antibodies recognized Mr 86,000, 62,000, and 101,000 molecules, each composed of a single polypeptide chain. The expression of these transformation-associated antigens was negligible on parental WFB cells. Transforming growth factor-beta could enhance the expression of all of these transformation-associated antigens, but platelet-derived growth factor could only enhance the Mr 86,000 kd molecule expression. In the cytotoxicity assays, poly-I:C-induced rat splenic NK cells were cytotoxic to W14 and W31, but not to WFB. The data also showed that the cytotoxicity by these NK cells against NK-sensitive YAC-1 cells was absorbed with the addition of W14, W31, platelet-derived growth factor, or transforming growth factor-beta-stimulated WFB cells. This indicates that NK cells may recognize common target antigens that are expressed among these target cells. It was also indicated that Mr 86,000 and 62,000 molecules were strongly involved in this cytotoxicity, possibly as the target antigens, since F(ab')2 fragments of monoclonal antibodies 109 and 061 strongly inhibited the cytotoxicity. The addition of monoclonal antibody 109, but not 061, inhibited the cytotoxicity even at 60 min after mixing with the effector and target cells, suggesting that the Mr 86,000 molecule may participate in the lethal hit phase of cytotoxicity by NK cells. These data may indicate that some, but not all, transformation-associated antigens are virtually important in the antitumor surveillance mechanisms by the host effector cells, such as NK cells.