Cell surface molecules of rat normal lymphoid cells were selectively labelled by lactoperoxidase catalysed iodination or by a galactose oxidase tritiated sodium borohydride technique, subsequently detergent solubilized, isolated by indirect immunoprecipitation and analysed on SDS-polyacrylamide gel electrophoresis. Four polypeptide chains were isolated by using the alloantiserum DA anti-Lewis. The molecular weights of the antigens were calculated as 41,000, 33,000, 27,000 and 12,000. Based on functional in vitro characteristics of the antiserum used and on the physiochemical properties as well as genetics of inheritance and tissue distribution, the polypeptide chains were identified as being subunits of Ag-B and Ia antigens. Two types of immunoglobulin heavy chains exhibiting the molecular weight 70,000 and 64,000 were isolated from unfractionated normal spleen cells by use of a polyvalent rabbit anti-rat immunoglobulin serum and tentatively identified as mu and delta chain. Using the same anti-immunoglobulin serum, no molecules could be precipitated from the lysated of Lewis thymocytes or peripheral T cells.