The present study was carried out to determine the conversion factor of synthetic (2)H-labelled beta-carotene to vitamin A in Chinese adults by using a stable-isotope dilution technique. Fifteen healthy volunteers aged 50-60 years were recruited for a 55 d experiment. The volunteers (nine males and six females) were each given a physiological dose of [(2)H8]beta-carotene (6 mg) in oil on the first day of the experiment, and a reference dose of [(2)H8]retinyl acetate (3 mg) in oil was given on the fourth day. Serum samples were collected at 0, 3, 5, 7, 9, 11, and 13 h on the first and the fourth days of the study, daily for 10 d, and then weekly from days 14 to 56. beta-Carotene and retinol were extracted from serum and isolated by HPLC, and their enrichments were respectively determined by using GC-electron capture negative chemical ionisation-MS and LC-atmospheric pressure chemical ionisation interface-MS. Four of the subjects exhibited beta-carotene to vitamin A conversion factors of >29.0:1 on a molar basis and were termed 'poor converters'. In the eleven normal converters (seven males and four females), the calculated conversion factors of beta-carotene to retinol ranged from 2.0:1 to 12.2:1 with an average of 4.8 (sd 2.8):1 on a molar basis, and from 3.8:1 to 22.8:1 with an average of 9.1 (sd 5.3):1 on a weight basis. The 52 d post-intestinal absorption conversion was estimated to be about 30 % of the total converted retinol.