Namirotene [CBS-211A] is a retinoic acid analogue which was undergoing phase II trials with Chauvin in France as 0.02% eye drops for corneal ulcer. It was discontinued later. Namirotene is a synthetic analogue of retinoic acid with differentiation inducing and potential antineoplastic activities. Like other retinoic acid agents, namirotene binds to and activates retinoic acid receptors (RARs), thereby altering the expression of certain genes leading to cell differentiation and decreased cell proliferation in susceptible cells. When it was associated with 1 alpha,25(OH)2D3, CBS-211A strongly potentiated the 1 alpha,25(OH)2D3-induced inhibition of U937 cell proliferation and caused a dramatic increase in their differentiation toward monocytes/macrophages. The co-inducing effect of CBS-211A was restricted to U937 cells. Our data suggest that CBS-211A may have therapeutic implications in the treatment of certain kinds of myelomonocytic leukemia. CBS-211A also provides an interesting tool to understand the mechanisms involved in the differentiation of myelomonocytic cells.