The effects of all trans retinoic acid (ATRA) on human myeloma cells growth were studied in vitro and in vivo using immunodeficient mice engrafted with malignant plasma cells. ATRA inhibited the in vitro proliferation of plasma cells originating from two patients with multiple myeloma whereas it had no effect on the in vivo growth of plasma cell grafts as assessed by the serial study of human Ig levels in mouse serum. Thus, the efficacy of ATRA for the treatment of human multiple myeloma remains to be ascertained.