Maize with enhanced provitamin A carotenoids (biofortified), accomplished through conventional plant breeding, maintains vitamin A (VA) status in Mongolian gerbils (Meriones unguiculatus). Two studies in gerbils compared the VA value of b-cryptoxanthin with b-carotene. Study 1 (n 47) examined oil supplements and study 2 (n 46) used maize with enhanced b-cryptoxanthin and b-carotene. After 4 weeks' depletion, seven or six gerbils were killed; remaining gerbils were placed into weight-matched groups of 10. In study 1, daily supplements were cottonseed oil, and 35, 35 or 17·5 nmol VA (retinyl acetate), b-cryptoxanthin or b-carotene, respectively, for 3 weeks. In study 2, one group of gerbils was fed a 50 % biofortified maize diet which contained 2·9 nmol b-cryptoxanthin and 3·2 nmol b-carotene/g feed. Other groups were given equivalent b-carotene or VA supplements based on prior-day intake from the biofortified maize or oil only for 4 weeks. In study 1, liver retinol was higher in the VA (0·74 (SD 0·11) mmol) and b-cryptoxanthin (0·65 (SD 0·10) mmol) groups than in the b-carotene (0·49 (SD 0·13) mmol) and control (0·41 (SD 0·16) mmol) groups (P,0·05). In study 2, the VA (1·17 (SD 0·19) mmol) and maize (0·71 (SD 0·18) mmol) groups had higher liver retinol than the control (0·42 (SD 0·16) mmol) group (P, 0·05), whereas the b-carotene (0·57 (SD 0·21) mmol) group did not. Bioconversion factors (i.e. 2·74 mg b-cryptoxanthin and 2·4 mg b-carotene equivalents in maize to 1 mg retinol) were lower than the Institute of Medicine values. Carotenoids comprise an important class of phytochemicals in plants. Among more than 600 naturally occurring carotenoids, a-carotene, b-carotene and b-cryptoxanthin are the major provitamin A carotenoids present in foods. The bioconversion rate of b-cryptoxanthin, a mono-hydroxylated xanthophyll found in tangerine, papaya, apricot, and red pepper, to vitamin A (VA) is not known. Efforts to biofortify foods such as carrots and maize (i.e. increase the provitamin A content) have focused on b-carotene (1,2) ; however, some varieties of biofortified maize contain substantial amounts of b-cryptoxanthin (2) . In order to direct maize-breeding efforts, it is important to know the contribution of b-cryptoxanthin to VA status.The structure of b-cryptoxanthin implies that one molecule will yield one molecule of retinol (Fig. 1). Unlike the hydrocarbons, a-and b-carotene, b-cryptoxanthin is bipolar because of the electronegative oxygen in the hydroxyl group on one side of the molecule. The effects of this bipolar structure on incorporation into mixed micelles and its impact on bioavailability have not been studied. As proposed by the Institute of Medicine (IOM) (3) , the bioconversion of dietary b-cryptoxanthin to VA is reported to be the same as a-carotene, i.e. 24 mg b-cryptoxanthin or a-carotene to 1 mg retinol (3) . This is twice the amount of b-carotene, i.e. 12 mg b-carotene to 1 mg retinol, and is based on the theoretical yield from the chemical structure. The IOM bioconversion factors were calcu...