A group of 30 coal-tar workers was treated with 1 g of ascorbic acid (AA) orally five times a week for 3 months. The effect of this treatment was assessed on serum IgG, IgM, IgA, alpha 1-antitrypsin, prealbumin, orosomucoid, transferrin, alpha 2-macroglobulin, C-reactive protein, ceruloplasmin, the latex fixation test and cancer serum index (CSI). After 3 months treatment the concentration of AA in the blood increased from 9.52 to 60.75 mumol l-1 (i.e. from 0.15 to 1.07 mg 100 ml-1), prealbumin increased from 0.37 +/- 0.08 g l-1 to 0.48 +/- 0.08 g l-1 (P less than 0.01), CSI decreased from 2.28 +/- 0.88 to 1.76 +/- 0.50 (P less than 0.01) and alpha 2-macro-globulin decreased from 3.40 +/- 0.95 to 2.06 +/- 0.39 g l-1 (P less than 0.01). These findings, together with reports that AA is a strong stimulator of xenobiotics biotransformation in the liver, support the use of AA as a prophylactic agent for coal-tar exposed workers.