Haptoglobin (Hp) is one of the acute-phase proteins and is mainly synthesized in the liver. During our study on the differentiation of leukemia cells, we have found that Hp is synthesized in human monocytic cells by all-trans retinoic acid (ATRA). The synthesis of Hp by ATRA is induced in a dose- and time-dependent manner, Hp cDNA cloned from ATRA-treated THP-1 cells corresponds to the Hp alpha2(FS)-beta form. Whereas ATRA acted as a strong inducer in THP-1 cells, IL-1 beta, TNF-alpha, IL-6, and LPS had little effect on Hp gene expression in these cells, These findings suggest that THP-1 cells express the Hp gene through a signal pathway different from hepatocytes, and that ATRA is a potent Hp-inducer in these cells. (C) zool Academic Press.