Abstract: The L1210 cell rapidly ceased to proliferate and its mitotic index decreased markedly after being exposed to HH07A 2μg·ml^-1 during exponential growth phase. However,34.6% of the cells were still able to form colonies in soft agar if HH07A was removed after 24 h ofincubation( the colony-forming efficiency for control cells was 63.7%)and the cell viabilityremained at about 94%.The DNA contents in L1210 cells were measured with a flow-cytometer.Theresults showed that the cell-cycle was blocked at the S phase and the number of cells in G₂+Mphasedecreased significantly.The time response course for L1210 cells indicated that the inhibitory effect of HH07A on L1210 clonogenic cells was slightly time dependent.