Cyclooxygenase and human tumor cell growth inhibitory extracts of horseradish (Armoracia rusticana) and wasabi (Wasabia japonica) rhizomes upon purification yielded active compounds 1-3 from horseradish and 4 and 5 from wasabi rhizomes. Spectroscopic analyses confirmed the identities of these active compounds as plastoquinone-9 (1), 6-O-acyl-beta-d-glucosyl-beta-sitosterol (2), 1,2-dilinolenoyl-3-galactosylglycerol (3), linolenoyloleoyl-3-beta-galactosylglycerol (4), and 1,2-dipalmitoyl-3-beta-galactosylglycerol (5). 3-Acyl-sitosterols, sinigrin, gluconasturtiin, and phosphatidylcholines isolated from horseradish and alpha-tocopherol and ubiquinone-10 from wasabi rhizomes isolated were inactive in our assays. At a concentration of 60 microg/mL, compounds 1 and 2 selectively inhibited COX-1 enzyme by 28 and 32%, respectively. Compounds 3, 4, and 5 gave 75, 42, and 47% inhibition of COX-1 enzyme, respectively, at a concentration of 250 microg/mL. In a dose response study, compound 3 inhibited the proliferation of colon cancer cells (HCT-116) by 21.9, 42.9, 51.2, and 68.4% and lung cancer cells (NCI-H460) by 30, 39, 44, and 71% at concentrations of 7.5, 15, 30, and 60 microg/mL, respectively. At a concentration of 60 microg/mL, compound 4 inhibited the growth of colon, lung, and stomach cancer cells by 28, 17, and 44%, respectively. This is the first report of the COX-1 enzyme and cancer cell growth inhibitory monogalactosyl diacylglycerides from wasabi and horseradish rhizomes.