Technical Abstract: Tomato plants (Lycopersicon esculentum) synthesize the glycoalkaloids dehydrotomatine and a–tomatine, possibly as a defense against bacteria, fungi, viruses, and insects. We investigated six green and three red tomato extracts for their ability to induce cell death in human cancer and normal cells using a microculture tetrazolium (MTT) assay. Compared to untreated controls, the high-tomatine green tomato extracts strongly inhibited the following human cancer cell lines: breast (MCF-7), colon (HT-29), gastric (AGS), hepatoma (liver) (HepG2), as well as normal human liver cells (Chang). There was little inhibition of the cells by the three low-tomatine red tomato extracts. We also evaluated cell death induced by the pure glycoalkaloids dehydrotomatine and a-tomatine isolated from green tomatoes and characterized by HPLC, GC, and GC/MS, as well as their respective aglycones tomatidenol and tomatidine. a-Tomatine was highly effective in inhibiting all the cell lines. Dehydrotomatine, tomatidenol, and tomatidine had little, if any effect on cell inhibition. The results show that the susceptibility to destruction varies with the nature of the alkaloid and plant extract and the type of cancer cell. These findings extend related observations on the anticarcinogenic potential of glycoalkaloids and suggest that consumers may benefit by not only eating high-lycopene red tomatoes, but also green tomatoes containing glycoalkaloids. Possible mechanisms of the anticarcinogenic and other beneficial effects and the significance of the cited observations for breeding improved tomatoes and for the human diet are discussed.