Metastases of ESb lymphoma cells in syngeneic DBA/2 animals frequently are selectively immunoresistant to lysis by syngeneic cytotoxic T-lymphocytes (CTL). This immunoresistance of tumor cells to CTL lysis could be due to a defect in either of two structures: (a) tumor-associated transplantation antigens; or (b) mouse major histocompatibility complex (H-2) antigens serving as restricting elements. In this study, we have analyzed the possible involvement of major histocompatibility complex Class I antigens in the immunoresistance of ESb tumor variant cells. Syngeneic anti-ESb CTL appeared to be H-2Kd restricted since only antibodies to Kd but not D,Ld molecules could inhibit CTL lysis. Comparison of H-2 antigens expressed on immunosensitive and resistant ESb sublines by immunofluorescence and flow cytofluorography, alloreactive CTL, two-dimensional gel analysis did not reveal any differences either qualitatively or quantitatively. Southern blotting of tumor-derived DNA with H-2-specific probes did not reveal differences either. Serologically detectable cell surface differentiation antigens were expressed very similarly on immunosensitive and resistant tumor lines, and only minor differences were noted by biochemical analysis of plasma membrane proteins. C-Type viral Mr 70,000 glycoprotein antigens were also similar in both types of cells. We conclude that cell surface changes on immunoresistant ESb variant cells are very selective and involve only CTL-defined tumor-associated transplantation antigen determinants.