We studied the activity of 14 compounds, all of which have been shown to interfere in plant cell division, in two animal tumor cell cultures, EL-4 and L1210. Four compounds [propham, chlorpropham, bensulide S-(O,O-diisopropylphosphorodithioate) ester of N-(2-mercaptoethyl)benzenesulfonamide), and siduron] had a 50% inhibitory dose less than 10(-4) M; six [2,3,5-triiodobenzoic acid, (2,4-dichlorophenoxy)acetic acid, bromacil, (2,4,5-trichlorophenoxy)acetic acid, naptalam, and (4-chloro-2-methylphenoxy)acetic acid] had a 50% inhibitory dose between 10(-4) and 10(-3) M, and the remaining four 2,3:4,6-di-O-isopropylidene-2-keto-L-gulonate, eptam, maleic hydrazide, and 4-(methylsulfonyl)-2,6-dinitro-N,N,-dipropylaniline] had a 50% inhibitory dose at higher than 10(-3) M. There was a significant correlation between the effect on the two cell lines as well as between the inhibition of cell proliferation and that of thymidine and leucine uptake. More detailed study of cell proliferation and leucine and thymidine uptake for bensulide and 2,3,5-triiodobenzoic acid revealed a dose-response pattern of inhibition starting shortly after exposure of the cells to the compounds. These results indicate that some inhibitors of plant cell division are capable of inhibiting the proliferation of animal tumor cells.