Quantitative studies have been made of Fc receptors on human leukocytes derived from peripheral blood, thymus, tonsil, and spleen. The relative affinities and average numbers of receptors per cell were determined by measuring the binding of 125I-labeled, affinity cross-linked trimers of rabbit IgG to various populations of cells. In parallel, the sizes of receptor-bearing populations were determined by fluorescence microscopy. Fc receptors could be detected on leukocytes from peripheral blood and spleen, but not from tonsil or thymus. In the peripheral blood, the highest density of receptors was found on polymorphonuclear leukocytes; a subpopulation of lymphocytes had somewhat fewer receptors per cell, and circulating monocytes had the lowest receptor density. Among splenocytes, most of the receptors were found on myeloid cells and monocytes. In all populations, the affinity of Fc receptors for the trimer was about the same. At 0 degrees C the average value for the association constant was 5 x 10(7) M-1.