Okadaic acid, a protein phosphatase inhibitor, is a strong tumor promoter which activates protein phosphorylation. Because another activator of protein phosphorylation, phorbol esters, stimulates hematopoietic differentiation, we sought to determine whether okadaic acid could also induce the differentiation of the human leukemic cell line U937. Differentiation was assessed by measuring changes in the following: mRNA levels, cell growth, morphology, cell surface markers, and the ability to induce superoxide. We found that okadaic acid treatment of U937 cells induces immediate increases in total cellular levels of both c‐junand c‐fos mRNAs. Nuclear run‐on experiments demonstrate that initial increases are secondary to increases in transcription, whereas latter changes may be secondary to mRNA stabilization. Like phorbol esters, okadaic acid treatment also activates AP‐1 enhancer activity and induces the phosphorylation of c‐Jun protein. Approximately 6–12 hours after treatment with okadaic acid, mRNA levels of c‐myc, p34cdc2, and p58GTA, two cell cycle regulated protein kinases, decrease. Okadaic acid inhibits the growth of U937 cells, induces changes in nuclear morphology, stimulates increases in Mac‐1 and Leu 11 surface antigens, and induces these cells to produce superoxide. These changes, taken together, suggest that U937 cells have been induced by okadaic acid to differentiate towards a more mature cell type. © 1992 Wiley‐Liss, Inc. Copyright © 1992 Wiley‐Liss, Inc.