Mitotic index (MI), defined as the ratio (%) of the mitotic-phase-arrested cell number to the total cell number, is higher in the presence of colcemid than in its absence. MI is reduced by anti-tumor agents used in conjunction with colcemid. The ratio of hll, which is defined by the dose of colcemid to that of the supplement with etoposide as an anti-tumor agent and is designated MIR, was 10% at an exposure of 2 h, suggesting that etoposide blocks the cell, cycle before 2 h of the mitotic phase. Thus, we established a convenient method to identify a target point in the cell cycle for anti-tumor agents using colcemid. RBL-2H3, a mammalian cell line, exhibited a large number of 2-fold swollen nuclear envelopes after exposure to etoposide. The G(2)-index, which is the ratio of the number of swollen nuclei to the total number of nuclei, was higher when the cells were incubated with etoposide than without it. In this study, we suggest that the measurement of the G(2)-index contributes to the screening of putative anti-tumor agents.