Normal healthy volunteers were studied after they ingested various beta-carotene doses. Daily administration of 15 or 45 mg beta-carotene resulted in significant increase in plasma beta-carotene levels. The extent of increase and the pattern of plasma beta-carotene levels showed substantial interindividual variation. Absorption of beta-carotene was affected by dietary fat concentration. Individuals placed on a high-fat diet showed significant increases in plasma beta-carotene as compared with those placed on a low-fat diet. Pharmacological doses of beta-carotene (45 and 90 mg) were used in intermittent schedules (5-6 d intervals) without altering the steady state of beta-carotene plasma levels. Yellowing of the skin occasionally occurred during daily dosing with 45 mg beta-carotene without evidence of toxicity. The observed individual variation in bioavailability of beta-carotene raises questions regarding clinical use of this micronutrient. It appears that determination of target plasma beta-carotene concentrations is essential for effective use of this compound in prevention or treatment.