A protein with an apparent m.w. of 68,000 has been observed in immunoprecipitations of NP-40 solubilized BW5147 thymoma cells by using monoclonal or polyclonal allospecific H-2K(k) antisera. By contrast, H-2D(k) alloantisera precipitated a 45,000 m.w. protein from the same membrane preparations. The 68,000 m.w. protein is expressed on the plasma membrane as determined by vectoral labeling. Extensive clearing experiments and the use of monoclonal antibody demonstrate that the protein does not bear group-specific C-type viral determinants.