The root extract of Trichosanthes cucumerina L. and bryonolic acid (1), its main constituent, as well as the fruit juice and cucurbitacin B (3), its main constituent, were tested for cytotoxicity against four human breast cancer cell lines (SKBR3, MCF7, T47D, and MDA-MB435), two lung cancer cell lines (A549 and SK-LU1), and one colon cancer cell line (Caco-2). The root extract had higher IC (50) values than bryonolic acid (1) against three breast cancer cell lines (MCF7 = 267/121, T47D = 316/124, MDA-MB435 = 140/90 microL/mL) and one lung cancer cell line (A549 = 106/100 microL/mL). The fruit juice also had higher IC (50) values than cucurbitacin B (3) against the four breast cancer cell lines (131/73, 375/35, 249/60, and 156/26 microL/mL, respectively) and one lung cancer cell line (141/41 microL/mL) as shown above, as well as against the colon cancer cell line (101/1.5 microL/mL). However, the root extract inhibited SK-LU1 more strongly than did the fruit juice, cucurbitacin B (3), and bryonolic acid (1) (149/169/180/>500 microL/mL, respectively). The root extract inhibited the two lung and three breast cancer cell lines (SKBR3, MDA-MB435, and MCF7) more strongly than the fruit juice. Bryonolic acid (1) inhibited MDA-MB435 somewhat better than the other tested human cancer cell lines. The fruit juice inhibited the colon cancer cell line (Caco-2) more strongly than the root extract. Cucurbitacin (3) inhibited human cancer cell lines, especially Caco-2, much more strongly than bryonolic acid (1). In addition to bryonolic acid (1), bryononic acid (2), cucurbitacin B (3), and dihydrocucurbitacin B (4) also were isolated from the root extract.