The effect of bleomycin A5 (BLM) alone and along with calmodulin inhibitor N-(4-aminobutyl)-5-chloro-2-naphthalene sulfonamide (W-13) on the proliferation of S-180 cells in vitro were studied. IC50 of BLM alone to the cells was about 2.63 micrograms/ml, which was decreased to 1/3.8 and 1/9.5 of 2.63 micrograms/ml when plus W-13 1, 5 micrograms/ml respectively. The results indicated that nontoxic doses of W-13 enhanced the inhibition of cell proliferation under the condition of BLM 0.5-2.5 micrograms/ml. In colony forming test, the survival fraction of S-180 cells treated with BLM plus W-13 was decreased to 1/87-240 of the cells treated with BLM alone. The results suggest that W-13 can enhance antitumor activity of BLM in vitro and may be used as an enhancer of BLM in vivo.