The purpose of the present study is to develop in vitro assays for rapid screening of a large number of food samples that contain components that prevent tumor formation in vivo and to identify the components that contribute to this antitumor effect. Wheat samples representing numerous strains and cultivars were screened for their in vitro ability to kill a human colon cancer cell, CaCo2, in culture by trypan blue dye exclusion assay. Wheat samples were assayed for orthophenolic acid content by use of a colorimetric assay using a bathochromatic shift at 350 nm. Blood levels of specific orthophenols were determined by high-performance liquid chromatography/mass spectrometry. Wheat samples, which contained low, mid-, and high in vitro protective ability, were used to formulate balanced diets fed to Min mice. Wheat samples with high ability to kill CaCo2 cells in culture had high levels of orthophenolic acids and produced elevated blood caffeic acid levels when used in diets. These factors correlated positively with their ability to prevent tumor formation in Min mice. When fiber content was equal in diets the content of orthophenolic acids in wheats predicted the antitumor activity in vivo.