The expression of Fc gamma R on subsets of mouse spleen cells was examined by dual parameter flow microfluorometry. B cells were detected by labeling them with antibodies against sIgM, sIgD, sIgG, or I-A; essentially all B cells expressed Fc gamma R. The number of Fc gamma R per cell on the sIgD+, sIgM+, and I-A+ cells averaged 2 X 10(4) receptors, and no correlation between the levels of expression of Fc gamma R and the B cell markers was evident. The sIgG+ B cells, however, expressed more Fc gamma R (8 X 10(4) receptors/cell) than sIgM+ and sIgD+ B cells. Fc gamma R on splenic macrophages were examined by double labeling spleen cells for Fc gamma R and Mac-1. The Mac-1+ cells (2 to 16% of the spleen cells) were 100% Fc gamma R+ and expressed threefold to fivefold higher numbers of Fc gamma R per cell than the sIgM+ or sIgD+ B cells. The Fc gamma R on T cells were studied on cells double labeled for Fc gamma R and Thy-1, Lyt-1, or Lyt-2. An average of 20% of the T cells expressed Fc gamma R and at least two subsets of Fc gamma R+ T cells were evident: Lyt-2- cells, most of which expressed intermediate (2 X 10(4) Fc gamma R/cell) levels of Fc gamma R, and Lyt-2+ cells, which expressed mainly high (8 X 10(4) Fc gamma R/cell) amounts of Fc gamma R. The levels of expression of Fc gamma R and sIgM increased dramatically in response to infection and were elevated in mice with genetic defects. We conclude that the level of Fc gamma R expression is a characteristic property of subsets of spleen cells from normal and infected mice.