Formaldehyde-fixed small lymphocytes from human tonsils were investigated for the presence of the Ig heavy chain class isotypes alpha, gamma, delta and mu on their membranes by means of a direct immunofluorescence technique. About 50% of the small lymphocytes were defined as B cells with an anti-Fab antiserum. Most of the B lymphocytes carried more than one isotype on their membrane, and all combinations of two isotypes were observed. The existence of three or four isotypes on the membrane of individual lymphocytes was deduced. The sIgD, sIgM and sIgA, sIgD, sIgM combinations were the most common. The isotypes were present as separate molecules since they showed independent distribution patients undeesent as separate molecules since they showed independent distribution patterns under capping conditions.