Several cell clones derived from cell lines obtained from a rat thyroid carcinoma, induced by in vivo injection of the Kirsten murine sarcoma virus into thyroid gland, and from its spontaneous lung metastases were analysed for their major histocompatibility complex (MHC) class I antigen expression. The susceptibility to natural killer (NK) cell lysis of these clones, differing in their levels of MHC class I antigen expression, was determined and found to vary inversely with the target cell MHC level, confirming numerous reports of the literature. We then tried to localize the step of the multistage natural cytotoxic process, in which class I antigens could interfere, and tested first whether lymphokine (IL-2) activation of the killer (LAK) cells could overcome the differences in MHC class I expression of target cells. As this did not appear to be the case, we studied the binding step by either a cold target inhibition assay and a target binding assay and found that target cells expressing class I antigens show a lower competitive capacity for effector cells than targets not expressing such antigens, indicating that this interference may occur, at least in our system, in the binding step of the cytotoxic process.