Epidemiological studies suggest that dietary limonoids and phytosterols offer protection from certain types of cancers. Potential cancer preventive constituents of sour orange (Citrus aurantium L.) were isolated and identified from ethyl acetate extract. The structures of the compounds were identified by one-dimensional (1H or 13C) and two-dimensional (1H-H and 1H-13C) nuclear magnetic resonance experiments as limonexic acid and beta-sitosterol glucoside (SG). The identified compounds were tested for the potential inhibition of human colon cancer cell (HT-29) proliferation, apoptosis, and also noncancerous cells (COS-1). Cell proliferation, arrest of the cell cycle, and induction of apoptosis were assessed by MTT assay, flow cytometry, and nuclear staining methods, respectively. The MTT assay indicated that both compounds exhibited significant inhibition at various concentrations. These compounds did not show any toxic effects on noncancerous cells. These compounds caused 4-5-fold increases in the counts of G2/M stage cells at 50 microM, indicating a potential role in cell cycle arrest. These findings support the hypothesis that limonoids and phytosterols are effective apoptosis-promoting agents and incorporation of enriched fractions of these compounds in the diet may serve to prevent colon cancer. To the best of our knowledge, this is the first report of the isolation, identification, and cell proliferation assay for limonexic acid and SG from sour orange.