Red cells sensitized with IgGl or IgG3 monoclonal anti-D antibodies were used in rosette assays with human lymphocytes, monocytes and granulocytes. With all three cell types, IgG3 antibodies promoted a greater degree of rosette formation than IgGl antibodies. Monocytes required a minimum of about 0.5 x 10^3 IgG3 molecules per red cell for rosette formation, and granulocytes and lymphocytes required around 1 x 10^4 IgG3 molecules per red cell. Approximately 80% of monocytes and granulocytes and 10% of lymphocytes were capable of rosette formation. These results are consistent with differences in the number and affinity of Fc receptors on different leucocytes. When compared with previous data these results suggest that binding of monocytes to monoclonal anti-D sensitized red cells is very similar to that of red cells sensitized with polyclonal antisera. Lymphocytes and granulocytes, however, appear to bind less well to red cells sensitized with certain monoclonal antibodies than with polyclonal antibodies. These findings may be of relevance to the prophylactic use of monoclonal anti-D antibodies.