The replication of several human and animal cancer cell lines is regulated in vitro and in vivo by 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3], the hormonally active form of vitamin D3. We have examined the effects of concentrations of 1,25-(OH)2D3, which inhibit cellular replication, on the cell-cycle kinetics of a 1,25-(OH)2D3-responsive human breast cancer cell line, T 47D. After 6 or 7 days of treatment, a time period representing approximately five cell population doublings of control cultures, concentrations of 1,25-(OH)2D3 in the range 10(-9) M to 10(-6) M caused a time- and concentration-dependent decrease in cell numbers. Treatment of cells growing in charcoal-treated fetal calf serum with 10(-8) M 1,25-(OH)2D3 for 6 days reduced cell numbers to 49% +/- 9% (n = 9) of control, and this was associated with a marked increase in the proportion of cells in the G2 + M phase of the cell cycle from 9.7% +/- 0.5% (n = 11) to 19.6% +/- 2.3% (n = 9), significant by paired analysis (P less than 0.002). At higher concentrations of 1,25-(OH)2D3 (10(-7)-10(-6) M), there was a concentration-dependent decline in S phase and increases in both G0/G1 and G2 + M phase cells. Detailed analysis of the temporal changes in cell-cycle phase distribution following treatment with 2.5 X 10(-8) and 10(-7) M 1,25-(OH)2D3 showed an initial accumulation of cells in G0/G1 and depletion of S phase cells during the first 24 hr of treatment. This decline in S phase cells was not accompanied by a decline in % G2 + M indicating a transition delay in G2 or mitosis. At the lower dose these changes returned to control values at 48 hr and at later times were associated with a slight but consistent decline in G0/G1 phase and an increase in G2 + M. In contrast cells treated with 10(-7) M 1,25-(OH)2D3 had significantly elevated % G0/G1 cells at days 2 and 3, consistent with a transition delay through G1 phase. This was confirmed in stathmokinetic experiments which demonstrated an approximate sevenfold decrease in the rate of exit of cells from G0/G1 following 4 days of exposure to 10(-7) M 1,25-(OH)2D3. This accumulation of cells in G0/G1 was accompanied by a fall in % S phase cells.(ABSTRACT TRUNCATED AT 400 WORDS)