Peripheral mononuclear cells from adult T cell leukemia (ATL) patients were analyzed in comparison with other types of leukemia cells, for the expression of transforming growth factor-beta (TGF-beta) mRNA, for the presence of TGF-beta activity (colony stimulating activity for normal rat kidney fibroblasts [NRK]) in conditioned medium and for their susceptibility to exogenous TGF-beta.