T-cell-mediated inhibition of granulomonopoietic progenitors (CFU-GM) was studied in vitro in 27 patients with severe aplastic anaemia (AA). In nine out of 13 responders to anti-thymocyte globulin (ATG), cultured T-cells obtained prior to therapy, as well as their conditioned medium strongly suppressed both normal allogeneic and autologous CFU-GM (the latter obtained after marrow recovery). Addition of anti-interferon-gamma to the cultures abolished the suppressive effect on CFU-GM. After ATG therapy, no similar inhibitory effect was detected. Employing the panning method with monoclonal antibodies (CD4+ for inducer/helper and CD8+ for cytotoxic/suppressor T-cells) we were able to show that the cells responsible for in vitro CFU-GM inhibition were included in the cytotoxic/suppressor T-cell subpopulation. Cultured T-cells and their conditioned medium obtained from 14 non-responders to ATG did not show CFU-GM suppression. The mean interferon (IFN) levels in the T-cell conditioned media of ATG-responders was 625 +/- 125 mu/ml while in non-responders the level was 45 +/- 15 mu/ml (normal control levels 43 +/- 24 mu/ml). Freshly isolated peripheral blood lymphocytes from either group did not show any in vitro inhibitory effect. The response rate to ATG was statistically significant when the generation in culture of high versus low IFN production was compared (P = 0.0001). Experiments with T-cells obtained from heavily transfused thalassaemia major, and myelodysplastic syndrome patients, as well as normal volunteers, also did not demonstrate any suppression of CFU-GM. Our results indicate that the response rate to ATG is significantly higher in patients with AA who have an abnormal regulation of interferon-gamma (g-IFN) production.