Guanylhydrazones, methylglyoxal bis(guanylhydrazone) (MGBG) and a new compound, phenylglyoxal bis(guanylhydrazone) (PGBG), interfering with polyamine biosynthesis have considerable potential for the use as antiparasitic and antitumor agents. The effect of these drugs on the cellular viability of Chinese hamster ovary cells was examined by in vitro neutral red assay. The time exposure and metabolic influence was studied. These compounds have a dose- and time-dependent cytotoxicity. The IC50 values were of 597.22 micrograms/ml and 1.77 micrograms/ml for MGBG after 3 and 24 h of incubation, respectively, and 380.50 micrograms/ml for PGBG after 24 h of incubation. The PGBG treatment during 3 h had no cytotoxic effect when the concentrations were lower than 3000 micrograms/ml. With the cytotoxicity assay used, we observed that the presence of S9 in cultured medium did not influence the cytotoxicity of these compounds.