A procedure was developed for the detection and determination of bixin and norbixin in human plasma by reversed-phase HPLC with a sensitivity limit of 5 µg l^- ¹. A group of seven volunteers ingested a single dose of 1 ml of a commercial Annatto Food Color (16 mg of cis-bixin in soybean oil). The presence of bixin (cis and trans) and norbixin (cis and trans) was demonstrated in the plasma at average levels of 11.6, 10.1, 2.8 and 0 µg l^- ¹ of bixin and 48, 58, 53 and 29 µg l^- ¹ of norbixin after 2, 4, 6 and 8 h, respectively. Considerable individual variations were observed. Complete plasma clearance generally occurred for bixin by 8 h and for norbixin by 24 h after ingestion of cis-bixin.