We observed the effects of the retinoic acid (13-cis retinoic acid; 13-cis RA, and all-trans retinoic acid; ATRA) for the cell growth and the expression of CD 25 on peripheral blood mononuclear cells (PBMC) from 17 patients with adult T cell leukemia (ATL). Fourteen had acute type, 1 had chronic type, and 2 had smoldering type of ATL. We divided those patient into 3 groups (hyper-sensitive, sensitive and resistant group) by determined with reduction rate of [3H]-thymidine incorporation obtained before and after treatment with 13 -cis RA or ATRA respectively. Growth inhibition was not observed in normal PBMC by 13 -cis RA or ATRA. However, no down-regulation of CD 25 expression was observed on PBMC in all patients and normal individuals after treatment with 13-cis RA or ATRA. In the aspect of growth inhibition on PBMC in ATL patients, we tried to clarify the mechanism of the phenomenon. In agarose gel electrophoresis, extracted genomic DNA from retinoic acid treated PBMC in hyper-sensitive and sensitive ATL patients showed multimer DNA fragmentation pattern. On the other hand, genomic DNA from PBMC after treatment with retinoic acid in resistant ATL patients and normal individuals showed high molecular DNA pattern without fragmentation. Taken together, it is suggested that retinoic acid could induce growth inhibition of PBMC in some ATL patients resulting in DNA fragmentation, apoptosis. We deeply consider that retinoic acid may be an useful agent for ATL patients in clinical aspect.