Cell surface expression of leukosialin (sialophorin, CD43 antigen) on human neoplastic hematopoietic cell lines K-562, U-937, HL-60 and REH was determined with the aid of a new CD43 monoclonal antibody (Bra7G) by the immunochemical (radioimmunoprecipitation, immunoblotting) and immunocytofluorometric techniques. Interferon-gamma and TNF-alpha were utilized as the "physiological" inducers of differentiation-associated markers. The "non-physiological" inducer phorbol ester PMA induced down-regulation of leukosialin cell surface expression on immature erythroid-myeloid leukemia cell line K-562, but up-regulation of CD43 antigen on the promyelocyte leukemia cell line HL-60 and, to a lesser extent on the monocyte-like U-937 and CALLA+ ALL cell line REH. Retinoic acid down-regulated leukosialin on both U-937 monocyte-like cells and the CALLA+ ALL cell line REH. In contrast to these data, interferon-gamma, TNF-alpha, retinoic acid and 1,25(OH)2-vitamin D3 induced the up-regulation of leukosialin in a promyelocytic leukemia cell line HL-60.