A simple procedure for conjugation of fluorescein isothiocyanate to bacterial dextran is described. Fluorescein conjugated dextran was used to detect dextran specific immunoglobulin M receptors on the surface of MOPC 104E tumor cells. The formation of fluorescent microprecipitate in the surrounding medium is required for detecting the antibody on the surface of the tumor cells. This procedure is used to estimate the number of tumor cells carrying IgM receptors in a mixed population. © 1973.