The physiologically active form of vitamin D3, 1,25-dihydroxy-vitamin D3 (1,25(OH)2D3), induces differentiation of several types of myeloid leukaemia cells. The acquisition of monocyte-like phenotype is accompanied by slower progression through the cell cycle, and G1 block has been reported to be the basis of this effect. It is shown here that human promyelocytic leukaemia HL60 cells treated with analogues of vitamin D3 which are potent inducers of monocytic differentiation have an additional cell cycle block. Exposure to 10(-7) M 1,25(OH)2D3 or 1,25-(OH)2-16-ene-D3 resulted in monocytic differentiation and the expected G1 block evident at approximately 48 h in a rapidly differentiating variant of HL60 cells (HL60-G), and at 96 h in the more slowly differentiating HL60-240 cells. In addition, a G2+M block was noted at approximately 72 h in HL60-G and HL60-240 cells. Exposure to vitamin D3 analogues also markedly increased the number of dikaryons, suggesting that cytokinesis was impaired more than karyokinesis. Treatment with a third analogue 25-hydroxy-16,23-diene-D3 produced little differentiation and had minimal effects on the cell cycle parameters. These findings indicate that vitamin D3 analogues regulate cell proliferation by control of the transition of G1 and G2+M phases, reminiscent of the cdc2/CDK2 type of cell cycle control.