Thein vitrochemosensitivity of three cancer cell lines [HT29 (colon), HeLa (cervical) and T47D (breast)] to eight synthetic tetrapeptides, analogs of AS-I toxin, with phytotoxic effect on a series of plants was studied. Mouse fibroblast L929 cell line was also tested for chemosensitivity to these peptides. All cell lines were especially sensitive to Cys-Val-Gly-Glu tetrapeptide with IC50values of 0.18, 0.3 and 0.63 mM for HT29, HeLa and T47D cells, respectively, whereas the IC50value for the L929 cells was higher than 1 mM. Antiproliferative activity was also observed with peptides Tyr-Val-Gly-Glu and His-Val-Gly-Glu with IC50values higher than those obtained for Cys-Val-Gly-Glu. For the rest of the peptides tested the IC50values were found close to or higher than 3 mM.