Zeaxanthin is a predominant xanthophyll in human eyes and may reduce the risk of cataracts and age-related macular degeneration. Spirulina is an algal food that contains a high concentration of zeaxanthin. In order to determine the zeaxanthin bioavailability of spirulina for dietary supplementation in humans, spirulina was grown in nutrient solution with 2H2O for carotenoid labelling. Single servings of 2H-labelled spirulina (4·0-5·0 g) containing 2·6-3·7 mg zeaxanthin were consumed by fourteen healthy male volunteers (four Americans and ten Chinese) with 12 g dietary fat. Blood samples were collected over a 45 d period. The serum concentrations of total zeaxanthin were measured using HPLC, and the enrichment of labelled zeaxanthin was determined using LC-atmospheric pressure chemical ionisation-MS (LC-APCI-MS). The results showed that intrinsically labelled spirulina zeaxanthin in the circulation was detected at levels as low as 10 % of the total zeaxanthin for up to 45 d after intake of the algae. A single dose of spirulina can increase mean serum zeaxanthin concentration in humans from 0·06 to 0·15 μmol/l, as shown in our study involving American and Chinese volunteers. The average 15 d area under the serum zeaxanthin response curve to the single dose of spirulina was 293 nmol × d/μmol (range 254-335) in American subjects, and 197 nmol × d/μmol (range 154-285) in Chinese subjects. It is concluded that the relative bioavailability of spirulina zeaxanthin can be studied with high sensitivity and specificity using 2H labelling and LC-APCI-MS methodology. Spirulina can serve as a rich source of dietary zeaxanthin in humans.