Molecular relationships between β2m and other cell surface antigens (H 2, Tla, Ia, and Thy 1) were studied with the double immunofluorescence method. Cells were incubated with an antiserum against one antigen, capped, and then tested with an antiserum against a second antigen. Capping of β2m on thymocytes led to simultaneous capping of H 2 and Tla but not Thy 1 antigens; capping of H 2 and Tla (but not Thy 1) antigens resulted in capping of all β2m detectable by the immunofluorescence method. Similarly, capping of β2m on B or T lymphocytes resulted in capping of H 2 and vice versa. Ia antigens on B lymphocytes were not capped after the redistribution of β2m. The authors conclude from these data that, in the cell membrane of thymocytes, virtually all the β2m molecules are associated with H 2 and Tla, but not with Thy 1, and that on the cell surface of T or B lymphocytes, virtually all β2m is associated with H 2 but not with Ia. No evidence of any significant free β2m on either thymocytes or splenocytes was found.