The stimulation of peripheral blood mononuclear cells by purified autologous and/or allogeneic monoclonal IgG was studied in five patients with multiple myeloma (MM), nine patients with monoclonal gammopathy of undetermined significance (MGUS) and six healthy individuals. Single cells secreting IFN-gamma or IL-2 were identified using an enzyme-linked immunospot assay. Patients' cells were preferentially stimulated by autologous monoclonal IgG at low concentrations (1-100 pg/ml), while 100 ng/ml or higher stimulated T cells both from patients and, to a lesser degree, healthy individuals. This biphasic dose-response of T-cell stimulation by autologous monoclonal IgG was reproduced in all patients. The numbers of cells secreting IFN-gamma and IL-2 in response to allogeneic IgG were significantly lower than the numbers obtained using autologous IgG in patients with MM and MGUS. Cells from healthy individuals were stimulated by allogeneic monoclonal IgG, but to a lesser extent. The results of this study support the presence of idiotype-reactive T cells in patients with MM and MGUS and also may suggest a general but less pronounced T-cell reactivity to monoclonal IgG among these patients.