The L1210 cells rapidly ceased to proliferate and its mitotic index decreased markedly after being exposed to HH07A 2 micrograms . 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 of incubation (the colony-forming efficiency for control cells was 63.7%) and the cell viability remained at about 94%. The DNA contents in L1210 cells were measured with a flow-cytometer. The results showed that the cell-cycle was blocked at the S phase and the number of cells in G2 + M phase decreased significantly. The time response course for L1210 cells indicated that the inhibitory effect of HH07A on L1210 clonogenic cells was slightly time dependent.