We analyzed changes in polyamine contents and the activities of biosynthetic enzymes during each phase of the cell cycle for a synchronized population of BY-2 cells. Based on our analysis of $H^3$-thymidine incorporation flow cytometry and the mitotic index, the M and $G_2$ phases seemed to occur at 8 h and from 2.5 to 8 h, respectively, after the release of aphidicolin. The respective activities of arginine decarboxylase (ADC), ornithine decarboxylase (ODC), and S-adenosyl methionine decarboxylase (SAMDC) at the beginning (7.4, 11.2, and 5.5 $nmol;mg^{-1};protein;h^{-1}$) were increased to 22.6, 22.1, and 15.1 $nmol;mg^{-1};protein;h^{-1}$. However, those increases do not coincide with the general change in polyamines reported from animal tells. In addition, the bi-phasic activation of polyamine biosynthetic enzymes, such as those found in the general animal model, was observed with ADC and ODC but not with SAMDC. These results suggest that the general animal model for explaining polyamine changes and SAMDC activation in the cell cycle cannot be applied to BY-2 cells. Further, our flow-cytometric analysis of cell populations may be a useful tool for evaluating the efforts of polyamines on tell cycle progression in BY-2 cells.