5,8,11,14-Eicosatetraynoic acid, a competitive inhibitor of arachidonic acid metabolism, rapidly and reversibly inhibited DNA synthesis in U937 cells. This inhibition was not due to cytotoxicity, as judged by studies with trypan blue, release of 51Cr-labeled proteins, and its reversibility. When cells were cultured in the presence of ETYA for several days, morphologic, enzymatic, and functional changes consistent with differentiation occurred. Morphologic evidence of differentiation was evident by light microscopy. The cells enlarged, the ratio of cytoplasm to nuclei increased, secretory granules and vacuoles developed, the apparent activity of nonspecific esterase rose, and ingestion of latex particles increased. A morphology consistent with that of an immature monocyte was evident by electron microscopy. These features included the development of lobulated nuclei, a reduced nuclear to cytoplasmic ratio, increased complexity and development of the cytoplasmic components, and the disappearance of fimbriated plasma membrane structures. In addition, putative polyribosomes were less evident. When cells differentiated by ETYA were cultured in media free of the inhibitor, DNA synthesis reinitiated and the cell number increased; differentiation was phenotypic and not genotypic. To examine whether ETYA-induced differentiation was obligatorily related to its suppression of DNA synthesis, cells were incubated in 50 μM hydroxyurea and DNA synthesis was inhibited for 24 to 36 h without morphologic evidence of cellular differentiation. However, addition of ETYA to cells prevented from dividing by hydroxyurea and subsequent culture for 72 h induced morphologic evidence of differentiation. The effects of ETYA on cell division and cell differentiation are closely related but can be dissociated. The molecular events underlying these results remain to be established.