Soluble transferrin receptors (sTfR) were detected in culture supernatants of activated human peripheral blood mononuclear cells (PBMC) using a sandwich ELISA technique with two non-cross-reacting TfR MoAbs. Mitogenic stimulation of lymphoid cells induced both up-regulation of TfR surface density and release of sTfR to the medium. Peak levels of sTfR in culture supernatants occurred at day 4 after activation, 1 day later than maximum expression of TfR in the plasma membrane. Production of sTfR was independent of proliferation, as demonstrated by measuring sTfR release by PBMC, which had been irradiated with a dose of 20 Gy before activation. In addition to these in vitro experiments, we tested the sera of 85 patients with systemic lupus erythematosus (SLE), an autoimmune disease accompanied by in vivo activation of lymphocytes, for their sTfR levels. No correlation of these data was detectable to serum concentrations of the soluble alpha-chain of the IL-2 receptor, an unequivocal marker of lymphocyte activation. However, they correlated negatively to the haemoglobin content of the patients' erythrocytes, indicating that erythroid progenitors are the predominant source of sTfR in SLE patients' sera.