AbstractLevels of transforming growth factor-β(TGF-β) activity in the conditioned medium of blood lymphocytes of twelve patients with aplastic anemia (AA), nine patients with myelodysplastic syndromes (MDS) and five normal volunteers were investigated. We were able to observe growth inhibitory activity on porcine endothelial cells only after acidification of the materials. The growth inhibitory activity is neutralized by anti-TGF-βantibody. It indicates that TGF-βexists as a latent form in the conditioned medium. On the basis of growth inhibition assay, the mean level of TGF-βproduction of MDS patients was estimated to be 188·199 pg/1·107cells and that of normal volunteers was 668·314 pg/1·107cells. In contrast, the lymphocytes of almost all of the AA patients failed to produce detectable amounts of TGF-β. No correlation between TGF-βlevels and peripheral blood parameters could be detected. Stimulation of lymphocytes by phytohemagglutinin is known to increase the production of TGF-β. Induction of TGF-βproduction was also observed in AA (45% of normal controls). Possible roles of the decreased production of TGF-βin the pathogenesis of AA were discussed.