The immunopharmacology and pharmacokinetics of synthetic beta-carotene were studied in complex clinico-laboratory investigations after single and repeated per os administrations of various doses of the drug to healthy volunteers and cancer patients. Repeated treatment with beta-carotene at a dose of 30 mg caused no significant changes in the immunological parameters studied. However, treatment with a dose of 250 mg altered the parameters as follows: alteration of beta-carotene content in blood, elevation of the counts of lymphocytes and T lymphocytes forming 'active' rosettes, increase in the proliferative response of lymphocytes to the mitogen PWM and activation of natural killers. Single administration of the drug in a dose of 1,000 and 2,000 mg led to short-term reversible immunodepression, appeared as leukopenia, abolition of the proliferative response of lymphocytes to mitogens and inhibition of T suppressors. Kinetics of retinoids and beta-carotene content in blood plasma was studied after single per os administration of drug megadoses.