Human T cell subpopulations have been defined on the basis of differential expression of either Fc receptors or specific cell-surface antigens. In this study, we utilized a series of monoclonal antibodies reactive with T cells, monocytes, and Ia antigens to characterize isolated subpopulations of T cells bearing receptors for the Fc portion of IgG (T gamma) and subpopulations of T cells bearing receptors for the Fc portion of IgM T mu. The results showed that the T mu population contained both inducer (OKT4+) and cytotoxic/suppressor (OKT5+) populations and was similar to the unfractionated T cell population, whereas the T gamma subset contained few T lymphocytes (OKT3+) and was not enriched for either T cell subset defined by these monoclonal antibodies. Rather, the T gamma population was comprised largely of Ia- cells possessing a monocyte antigen (OKM1+). In reciprocal studies, it was found that both isolated OKT4+ and OKT5+ T cell subsets contained few T gamma cells, whereas both subsets were mainly comprised of T mu cells. We conclude that there is little correlation between T cell subsets defined by these monoclonal antibodies and those defined by Fc receptors.