Various abnormalities of lymphokine production have been described in patients with aplastic anemia. To determine if abnormal production of hematopoietic growth factors could contribute to the process of aplastic anemia we studied the in vitro production of human granulocyte- macrophage colony-stimulating factor (GM-CSF) and interleukin-3 (IL-3) by phytohemagglutinin (PHA)- and antithymocyte globulin (ATG)- stimulated peripheral blood lymphocytes from 29 patients with aplastic anemia and 15 normal controls. GM-CSF production in response to 1% PHA was seen in nearly all samples (43 of 44) and similar amounts of GM-CSF were produced by patients with aplastic anemia and normal controls. Production of GM-CSF by ATG-stimulated lymphocytes was seen in 7 of 23 patients with aplastic anemia (30%); two of these patients also demonstrated low-level spontaneous production of GM-CSF. Production of GM-CSF in response to ATG was also seen in 2 of 11 normal controls (18%) and barely detectable spontaneous production of GM-CSF was seen in both. Biologically active IL-3 could also be detected in PHA- or ATG- stimulated peripheral blood mononuclear cells in several patients and normal controls. Our results indicate that lymphocytes from patients with aplastic anemia can be stimulated in vitro to produce normal quantities of GM-CSF, suggesting that impaired potential for production of T-cell derived hematopoietic growth factors is unlikely to account for the marrow hypoplasia seen. In several patients overproduction of GM-CSF was observed, consistent with the notion that some patients with aplastic anemia may have circulating activated T cells. We also demonstrate that ATG can stimulate the production of growth factors such as IL-3 and GM-CSF, supporting the role for ATG in stimulating hematopoiesis.