The covalently binding fluorescent probe 5-dimethylamino-1-naphthalenesulfonyl (dansyl) chloride was affixed directly to the plasma membrane of viable human peripheral blood lymphocytes via a solid phase transfer method utilizing Sephadex G-10 as the transfer vehicle. After dansylation, lymphocytes retain maximal short-term viability. Dansyl, as the protein conjugate or as the free acid, does not appear to penetrate the cells to any significant extent. Dansylated mixed lymphocyte cultures respond to lectin mitogen stimulation for at least 72 hr. Furthermore, differential response of dansylated lymphocytes in culture to three plant lectin mitogens provides a clue to the binding loci of concanavalin A with respect to phytohemagglutinin and pokeweed mitogen on the lymphocyte surface receptors for these lectins. The ability to sustain functionally responsive dansylated lymphocytes for several days in culture suggest that such probe-tagged cells may be useful in elucidating aspects of the plasma membrane in the regulation of cell behavior.