Human subjects (n = 24) were supplemented with 100 mg beta-carotene/d for 6 d, either as synthetic all-trans-beta-carotene or a natural beta-carotene preparation derived from the alga Dunaliella salina, which consists of a 50:50 mixture of all-trans- and 6-cis-beta-carotene. This loading dose was followed by a 23-d maintenance dose consisting of alternate-day supplementation with 50 mg all-trans-beta-carotene or either 66 or 100 mg of the natural 50:50 isomeric mixture. The loading dose resulted in significant increases in plasma concentrations of both isomers, with the all-trans-beta-carotene-supplemented group showing a 7.2- and 5.0-fold increase in the all-trans and 9-cis concentrations in plasma, respectively. The group receiving the 50:50 mixture showed a 4.0- and 3.7-fold increase in the all-trans and 9-cis concentrations in plasma, respectively, without any apparent dose-dependency. However, even with the 50:50 mixture, the 9-cis concentrations were only a small fraction of the total plasma beta-carotene. Results after an additional 23-d period of alternate-day supplementation were not significantly different from those described above for the 6-d supplementation. Increases in low-density-lipoprotein concentrations of total beta-carotene correlated strongly with the increases seen in plasma concentrations. Lipid-soluble antioxidants vitamin E and ubiquinol were unaffected by beta-carotene supplementation. However, the amount of lycopene in the low-density lipoprotein decreased during this supplementation period. A strong discrimination between these two geometric isomers of beta-carotene was demonstrated, although the tissue site of discrimination was not determined.