For many years, several studies have been employing lectin from vegetables in order to prove its toxic effect on various cell lines. In this work, we analyzed the cytotoxic, antiproliferative, and post-incubatory effect of pure tepary bean lectins on four lines of malignant cells: C33-A; MCF-7; SKNSH, and SW480. The tests were carried out employing MTT and 3[H]-thymidine assays. The results showed that after 24 h of lectin exposure, the cells lines showed a dose-dependent cytotoxic effect, the effect being higher on MCF-7, while C33-A showed the highest resistance. Cell proliferation studies showed that the toxic effect induced by lectins is higher even when lectins are removed, and in fact, the inhibition of proliferation continues after 48 h. Due to the use of two techniques to analyze the cytotoxic and antiproliferative effect, differences were observed in the results, which can be explained by the fact that one technique is based on metabolic reactions, while the other is based on the 3[H]-thymidine incorporated in DNA by cells under division. These results allow concluding that lectins exert a cytotoxic effect after 24 h of exposure, exhibiting a dose-dependent effect. In some cases, the cytotoxic effect is higher even when the lectins are eliminated, however, in other cases, the cells showed a proliferative effect.