The role of cell surface charges in the behavior of sheep red blood cells (SRBC) rosettes with human lymphocytes was investigated by using various polycations and polyanions. Polycations (DEAE-dextran, polybrene, poly-l-lysine, spermadine) at appropriate concentrations (1) invariably increased the proportions of early rosettes. This was so irrespective of whether it was the SRBC or the lymphocytes that were being treated with the reagents. (2) The rosettes so formed also consisted of a greater number of red blood cells per rosette (3). On incubating the rosettes at 37°C, they dissociated by capping. The rates of dissociation and capping were retarded by polycation treatment. (4) The resistance to rosette disruption was increased. These properties were very similar to those observed on cells treated with neuraminidase. Treatment of cells with the polyanions heparin and dextran sulfate induced the opposite effects: reduced the number of rosettes and increased the rate of dissociation, and decreased the resistance of rosettes to disruption. It was concluded that alterations of cell surface charges could drastically influence SRBC-lymphocyte interactions. To investigate whether rosette inhibitors could do so by altering cell surface charges, cell were treated with anti-lymphocytic sera, 2,4-dinitrophenol and sodium azide. All three agents inhibited rosette formation. Whereas DEAE-dextran could partially reverse the inhibition by the first two agents, it failed to do so with the third. This suggested that the first two may have inhibited rosette formation by altering the surface charges.