Peripheral blood mononuclear leukocytes (MNL) from 11 of 30 healthy adults elaborated detectable IgM RF when stimulated with pokeweed mitogen. In order to assess the possibility that suppressor T-cell activity might account for the nonexpression of IgM RF in vitro in some individuals, the influence of T cells on IgM RF production by autologous B cells prepared from donors whose unfractionated MNL synthesized IgM RF in response to PWM was investigated. Untreated T cells supported IgM RF production by autologous B cells with optimal synthesis observed at T:B cell ratios of 2:1: at higher T:B cell ratios a decline in IgM RF production occurred. In contrast, at higher T:B cell ratios irradiated T cells supported consistently higher levels of IgM RF production than untreated T cells suggesting the presence of radiosensitive suppressor T cells for IgM RF in these individuals. To determine the frequency of these suppressor cells, irradiated T cells were compared to untreated T cells for capacity to support IgM RF production by autologous B cells from 12 randomly selected donors at T:B cell ratios of 3:1. Untreated T cells from 4 of 12 individuals were capable of cooperating in induction of IgM RF production by autologous B cells, whereas irradiated T cells supported IgM RF production in 6 of 12 individuals. Levels of IgM RF production in all 6 individuals were significantly higher with irradiated T cells than with untreated T cells; in 2 individuals IgM RF synthesis by autologous B cells was observed only in the presence of irradiated T cells. In 4 of 6 individuals increases in the ratio of IgM RF total IgM synthesis occurred with irradiated T cells (when compared to untreated T cells), suggesting disproportionate suppression of RF production. © 1981, SAGE Publications. All rights reserved.