Summary: Lactoperoxidase‐catalyzed radioiodination of cell surface proteins was employed in the isolation of cell surface immunoglobulin from T cells activated by sheep red blood cells. Immunoglobulin was identified by specific coprecipitation with rabbit anti‐mouse immunoglobulin antiserum and mouse Immunoglobulin. Polyacrylamide gel electrophoresis of reduced and alkylated precipitates showed that the immunoglobulin molecules possessed light chains And μ‐type heavy chains. Cell surface immunoglobulin isolated from activated T cells possessed binding specificity for the activating antigens which paralleled the functional activity of the cells The results indicate that the cell surface Immunoglobulin of activated T cells can function as the cell receptor for antigen.