Deregulation of cyclin expression has been found in many tumors. In this report, we studied expression of cyclin DI in three human prostate cancer cell lines: the androgen-dependent LNCaP and the androgen-independent PC3 and DU 145 cell lines. Northern blot analysis showed that DU145 and PC3 cells expressed more abundant cyclin DI than LNCaP cells. Southern blot analysis showed no evident gene amplification or rearrangement of cyclin DI in any of these cell lines. Serum starvation and replenishment were used in the cell culture to study the regulation of expression of cyclin DI. Cyclin DI mRNA expression was detected by Northern blot analysis when LNCaP cells grew in medium with serum but was not detected after serum withdrawal; however, cyclin DI mRNA was induced after serum was added. Cyclin DI mRNA expression by PC3 and DU 145 cells was detected both when they grew in medium with serum and after serum withdrawal, although expression decreased greatly after 24 hours in the PC3 cell line. Immunoprecipitation and immunohistochemical staining also showed that cyclin D I protein was always expressed in PC3 and DU 145 cells under different growth factor environment, whereas it decreased significantly in LNCaP cells deprived of serum and the level resumed again when serum was re-added. This suggests that expression of cyclin DI is regulated by exogenous growth factors in LNCaP cell line and becomes constitutive in PC3 and DU 145 cell lines.