Diospyros kaki Thunb. (Ebenaceae) is widely distributed in North-East Asian countries. Almost all parts of this plant have been traditionally used as medicine. Human promyelocytic leukemia cells differentiate into monocytes or granulocytes when treated with 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] or all-trans retinoic acid (ATRA). Combination of low doses of ATRA or 1,25-dihydroxyvitamin D3 that do not induce toxicity with another drug is a useful strategy for acute promyelocytic leukemia therapy. Our main aim was to investigate the effect of an acetone extract of D. kaki leaves (KV-1) on HL-60 cell differentiation in combination of ATRA or 1,25-dihydroxyvitamin D3. Treatment of HL-60 cells with zero to 100 microg/ml of KV-1 for 72 h induced a small increase in cell differentiation. Surprisingly, a synergistic induction of differentiation was observed when the HL-60 cells were treated with ATRA or 1,25-(OH)2D3 and the extract. The inhibitors of protein kinase C (PKC) (alpha and betaI) and extracellular signal-regulated kinase (ERK), but not of phosphoinositide 3-kinase (PI3-K) and c-Jun N-terminal kinase (JNK) inhibited the HL-60 differentiation induced by the extract in combination of ATRA or 1,25-(OH)2D3, suggesting that PKC and ERK were involved in the cell differentiation enhancement by the extract. The results indicate that the acetone extract of D. kaki leaves has the ability to enhance HL-60 cell differentiation and suggest that it may be useful in acute promyelocytic leukemia therapy.