The inhibiting activity of serum on haematopoiesis has been described in patients with SLE. To explore further the features of serum inhibitor, we first examined the suppression of granulocytic and erythroid colony formation in vitro by serum from patients with SLE using methylcellulose culture. The potent inhibiting activity was demonstrated in six of 20 patients. All of these six patients were associated with leukocytopenia and/or anaemia. Five of 10 sera from patients with active SLE suppressed the colony formation of both burst-forming units of erythrocyte (BFU-E) and colony-forming units of granulocyte/macrophage (CFU-GM), and one serum suppressed BFU-E only. IgG fraction isolated from sera with inhibiting activity suppressed colony formation without complement involvement. The elimination of monocytes and lymphocytes from target mononuclear cells did not affect the suppression by the IgG fractions. The suppressive effect was completely eliminated after incubation of the IgG fractions with progenitor-enriched mononuclear cells. Flow cytometric analysis showed these IgG bound to CD34+ haematopoietic progenitor cells, but not to CD33+ cells. These data suggest that (i) the inhibitor of colony formation in serum was observed in IgG fraction; (ii) its suppressive effect on colony formation was mediated by neither monocytes and lymphocytes nor complements; and (iii) IgG fraction could bind to primitive haematopoietic progenitor cells and suppress the growth of these cells. Thus, IgG autoantibodies to primitive haematopoietic progenitor cells are demonstrated to be present in the sera of a significant proportion of active SLE patients with anaemia and leukocytopenia and to suppress the progenitor cell growth.