OBJECTIVE To determine the effect of vitamin D3 analogue (EB-1089) on the growth and proliferation of a prostate cancer cell line (PC-3). METHODS PC-3 cells (10(4) cells per well) were plated into 24-well tissue culture plates. After 24 h, the culture medium was replaced with one containing the vitamin D3 analogue EB-1089; a control treatment using only replacement medium was conducted in parallel. Cell proliferation was measured by the incorporation of 3H-thymidine 7 and 12 days after the addition of the vitamin D3 analogue. Cells were precipitated with 5% trichloroacetic acid and the radioactivity determined using a scintillation counter. Each experiment was performed at least five times. RESULTS There was a significant dose-dependent inhibition of cell growth after 7 and 12 days of treatment with EB-1089, varying from 40 to 70% of the 3H-thymidine incorporation by controls, respectively. The maximum inhibition occurred with 0.1 micromol/L EB-1089 on day 7 and day 12 (both P < 0.01). Longer incubation times appeared to have a greater effect when higher concentrations of EB-1089 were used. CONCLUSIONS These in vitro studies have shown that the vitamin D3 analogue EB-1089 can significantly reduce the growth rate of the prostate cancer cell line PC-3. This would support the hypothesis that deficiency of vitamin D increases the risk of prostate cancer and further in vivo testing of vitamin D is warranted for its potential role in active therapy.