The effects of anticalmodulin agents, namely trifluoperazine (TFP) and two naphthalene sulfonamide derivatives (W-7 and W-13), were tested on the growth of a human breast cancer cell line (MDA-MB-231) using a soft agar clonogenic assay. The results of this in vitro study reveal that TFP, W-7, and W-13 had the ability to inhibit the colony formation from this cell line. The inhibitory effect was greater when the cancer cells were exposed to these agents continuously than when the cells were exposed to the drugs for 1 h. The IC50 values for TFP, W-7, and W-13 in continuous exposure were about 18, 30, and 38 microM, respectively, whereas the corresponding values for 1-h exposure were 50, 53, and 70 microM, respectively. These findings suggest that anticalmodulin agents can inhibit the growth of human cancer cells at relatively low concentrations in vitro. Whether effective antitumor concentrations of these drugs can be achieved in vivo remains a subject for further study.