More information on the bioefficacy of carotenoids in foods ingested by humans is needed. Objective: We aimed to measure the time required for isotopic enrichment of β-carotene and retinol in serum to reach a plateau, the extent of conversion of β-carotene dissolved in oil with use of β-carotene and retinol specifically labeled with 10 13C atoms, and the intraindividual variation in response. Design: Indonesian children aged 8-11 y (n = 35) consumed 2 capsules/d, 7 d/wk, for ≤ 10 wk. Each capsule contained 80 μg [12,13,14,15,20,12′,13′,14′,15′,20′- 13C10]β-carotene and 80 μg [8,9,10,11,12,13,14,15,19,20-13C10] retinyl palmitate. Three blood samples were drawn per child over a period of ≤ 10 wk. HPLC coupled with atmospheric pressure chemical ionization liquid chromatography-mass spectrometry was used to measure the isotopic enrichment in serum of retinol with [13C5]retinol and [13C10]retinol and of β-carotene with [13C10] β-carotene. The β-carotene in the capsules used had a cis-trans ratio of 3:1. Results: Plateau isotopic enrichment was reached by day 21. The amount of β-carotene in oil required to form 1 μg retinol was 2.4 μg (95␌I: 2.1, 2.7). The amount of all-trans-β-carotene required to form 1 μg retinol may be lower. Conclusions: The efficiency of conversion of this β-carotene in oil was 27␋etter than that estimated previously (1.0 μg retinol from 3.3 μg β-carotene with an unknown cis-trans ratio). The method described can be extended to measure the bioefficacy of carotenoids in foods with high precision, requiring fewer subjects than other methods.