The proteins on surfaces of living splenic lymphocytes from normal BALB/c mice were iodinated enzymatically. Such cells were fractionated into two sub-populations: one composed almost exclusively of small lymphocytes and the other mainly of large lymphocytes and plasma cells. Specific immunoprecipitation of radiolabeled surface Ig obtained from lysates of these cell populations indicated that approximately 2-3% of the acid-precipitable radioactivity from the cell surface is Ig. Moreover, 95% of the H chain radioactivity from the Ig of the small lymphocyte fraction and 90% from the large lymphocyte-plasma cell fraction was characterized as micro by precipitation with anti-micro sera as well as by molecular weight determination on polyacrylamide gels in sodium dodecyl sulfate. The Ig was recovered from the cell surface in the form of an IgM monomer. Control experiments suggested that the monomer did not result from depolymerization of 19S IgM by the methods used to radiolabel and isolate the molecule. (3)H-tyrosine labeling of IgM produced by meyloma cells and radio-iodination of IgM in solution gave the same ratios of microL radioactivity as radiolabeling of IgM on cells, indicating that the tyrosine residues of L and micro-chains of cell surface IgM are available to the lactoperoxidase during the iodination. This is consistent with the hypothesis that cell surface IgM is entirely on the outside of the plasma membrane presumably attached to it by its Fc fragment. These results, together with previous reports by others, suggest that IgM, in its monomeric form, is the main antigen-specific receptor on lymphocytes of normal mice.