This preliminary study was undertaken to investigate the chemical nature of the Thy 1.2 antigen expressed on the murine cell line S 49.1 TB 2.3 (S 49). The presence of the Thy 1.2 antigen was indicated by the inhibition of AKR anti C3 Thy 1.2 serum induced lysis of 51Cr primed target cells. It was found that limited digestion of S 49 cells with crude papain yielded a Thy 1.2 containing solution. The protein nature of the Thy 1.2 antigen obtained in this manner was indicated by changes after proteolytic digestion. Separate digestions with crustalline papain, insolubilized papain, and insolubilized protease all destroyed the Thy 1.2 activity. These results suggest that the protein moiety is necessary for Thy 1.2 activity.