We previously reported that garlic grown in a selenium-fertilized medium (selenium-enriched garlic) is superior to regular garlic in mammary cancer prevention in an animal model (Nutr Cancer 17, 279-286, 1992). The present study was designed to evaluate the nutritional bioavailability of selenium from this garlic with use of two liver selenoenzymes as biomarkers: glutathione peroxidase and type I 5'-deiodinase. Rats were fed a selenium-deficient diet (0.01 ppm Se) from weaning for four weeks to deplete both enzymes. They were then supplemented with nutritional levels of selenium (0.1-0.5 ppm) in the form of sodium selenite (positive control) or selenium-enriched garlic. Our results showed that selenium-enriched garlic was just as effective as selenite in restoring the activity of both selenoenzymes. This was demonstrated in a time course repletion experiment as well as in a dose-response experiment. Thus the selenium in selenium-enriched garlic has potent nutritional and anticancer efficacy. The type I 5'-deiodinase enzyme catalyzes the conversion of thyroxine (T4) to 3,5,3'-triiodothyronine (T3) and is responsible for most of the circulating T3. Because cancer chemoprevention by selenium usually requires pharmacological levels of selenium, we also examined the possible modulation of type I 5'-deiodinase by long-term feeding of selenium-enriched garlic at 3 ppm Se in the diet. The observation that a high intake of selenium-enriched garlic did not affect 5'-deiodinase activity suggests that its anticarcinogenic effect is unlikely to be mediated by an imbalance in the blood T4-to-T3 ratio.