Overexpression of wild-type p53 gene in malignant cell lines has been shown to inhibit cell proliferation in a number of cases. However, endogenous p53 protein seems to play little role in normal cell-cycle control as suggested by the normal development of p53 null mice, and by the low p53 protein levels expressed in most cell types. Recently, increased expression of endogenous p53 protein has been observed during the cellular response to DNA damage, as well as during differentiation of human hematopoietic cells. To study the role of the p53 gene in hematopoietic differentiation, we introduced the wild-type p53 gene or the temperature-sensitive p53(Val135) mutant into p53-deficient HL-60 promyelocytic leukemia cells. Morphological analysis, flow-cytometric determination of granulocytic or monocytic surface markers, and ability to reduce nitroblue tetrazolium (NBT) demonstrated that expression of exogenous wild-type p53 gene in HL-60 cells induces differentiation through the granulocytic pathway. Proliferation and cell-cycle analysis performed early after expression of wild-type p53 showed that induction of differentiation is not coupled with growth arrest, which suggests that p53 is involved in differentiation independently of its activity on the cell cycle.