Fluorochrome-labeled, human immunoglobulins (Igs) were used to characterize peripheral blood mononuclear cell (PBMC) subsets that had Fc receptors (FcRs) for the three major isotype classes, IgG, IgA and IgM. Relatively high concentrations of these Igs were needed to saturate FcRs. However, by a number of criteria, competition assays in particular, isotype-specific FcR binding was shown. In vivo bound Ig was directly measured by anti-Fc class-specific F(ab)2 antibodies. With the exception of monocytes, where a mean of 19% had in vivo bound IgG, only small percentages of other PBMC subsets had any type of in vivo bound Ig. Studies done on a large group of normal individuals revealed that approximately 27% of all PBMCs had gamma, 24% alpha, and 23% microFcRs. Dual fluorescence assays were performed to determine the gamma, alpha, and microFcR status on T, B, NK cells and on monocytes. The majority of staining for gamma, alpha, and microFcRs was accounted for by the numerically largest subfraction, the T cells. However, when FcR status was determined for each subset virtually all had either gamma, alpha, or microFcRs--94% of CD2+, 98% of CD4+, 73% of CD8+, 100% of CD19+, 100% of CDw19+, 92% of Leu 7+, and 61% of Leu 11+ cells. All three FcRs were expressed within each subset. Of interest, a small percentage of unfractionated and monocyte-depleted PBMCs coexpressed one of three pairs of FcRs, either alpha and gamma, alpha and mu, or mu and gamma.