Retinol (vitamin A) in concentrations above 5 µg/ml inhibits the stimulation of human lymphocytes by phytohemagglutinin and phorbol myristate acetate. This is in contrast to bovine lymphocytes in which retinol inhibits only phorbol myristate acetate stimulation of lymphocytes. The cause of the inhibition does not appear to be due to a direct toxic action because the viability and ultrastructure of lymphocytes exposed to a concentration of 20 µg/ml for 72 hr were normal. The mechanism of action of the retinol is not known, but it may be mediated by a retinol-binding protein in the cells.

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This work was supported by a grant from the National Cancer Institute of Canada.

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