BALB/cBy (Qa-2-) mice injected with the syngeneic tumor, ORA I-a (Qa-2+), produced antibodies to Qa-2 and a newly discovered antigen, Qa-6. Specific antisera against Qa-6, in the presence of complement, lyses approximately 40% of lymph-node lymphocytes and splenocytes. Strain distribution analyses indicate that Qa-6 is specified by a gene within the TL subregion of the major histocompatibility complex. Thus, Qa-6 is the third member of the Qa/TL subset of cell surface antigens which is anomalously expressed on certain tumor cells. This finding suggests that the Qa and TL molecules may have a unique, functional role on the cell surface.