Since cis or trans isomers of carotenoids may have different biological reactivities, the isomeric composition of lycopene and beta-carotene was measured in serum and seven human tissues. In addition to all-trans lycopene, at least three cis-isomers (9-, 13-, and 15-cis) were present, accounting for more than 50% of total lycopene. 13- and 15-cis-beta-carotene, however, were present at only 5% of the all-trans isomer. In addition, 9-cis-beta-carotene was present in tissue samples but not in serum. There were interindividual differences in carotenoid levels of the different tissue types, but liver, adrenal gland, and testes always contained significantly higher amounts of the carotenoids than kidney, ovary, and fat; carotenoids in brain stem tissue were below the detection limit. beta-Carotene was the major carotenoid in liver, adrenal gland, kidney, ovary, and fat, whereas lycopene was the predominant carotenoid in testes.