It is widely assumed that during the mammalian cell cycle, four major cyclins, including G1 cyclins (D, E) and mitotic cyclins (A, B1), are expressed in an orderly scheduled pattern in exponentially cultured cells. In high-density cultured cells and in vivo growing cells, whether these cyclins are expressed in the same pattern remains unknown. In this study, we investigated the expression of cyclins by flow cytometry and western-blotting in cultured MOLT-4 and HepG2 cells at high-density. We found that, in high-density cultures, the expression levels of cyclin D, E, and A within each cell cycle phase was less than their levels in low-density cultures. Cyclin B1 was slightly reduced in G2/M phase and unexpectedly expressed in G1 phase. The cyclin B1 expressed in G1 phase was localized mainly in the cytoplasm, and its level was positively correlated with apoptosis. Moreover, we obtained similar cyclins expression pattern in inoculated HepG2 tumor cells from immune-deprived mice and leukemic cells from leukemia patients. These findings describe an altered pattern of cyclin expression in high-density cultured cells as well as in vivo tumor cells. We suggest that high-density cultures might mimic the in vivo growth conditions and such experimental situation may be useful for cell cycle research.