A retinol (15 mg), riboflavin (200 mg) and zinc (50 mg) supplement or a placebo (D-mannitol) was given to 610 Chinese farmers weekly for 13.5 months. Riboflavin, retinol, beta-carotene, tocopherol and zinc status were measured in all subjects at the start and end of the intervention and in a subsample at 2 months. Plasma vitamin C, haemoglobin and haematocrits were measured on other subsamples at the end of the supplementation period. At the start vitamin A, zinc and riboflavin status were poor. Plasma retinol and zinc concentrations were higher in men than women but the opposite was observed with respect to beta-carotene. There was no difference between the sexes for tocopherol and riboflavin. Tocopherol:cholesterol ratios suggested vitamin E status was good and vitamin C status also appeared adequate. The supplement increased plasma retinol throughout the whole period but riboflavin status improved only over the first 2 months and then remained constant. Plasma zinc increased in both treatment and placebo groups by approximately 15 to 20 per cent in men and women. Mean concentrations of retinol and tocopherol and beta-carotene (in women only) increased by approximately 20 per cent in the placebo group. It is argued that seasonal factors and/or the placebo may have been responsible for the increase in retinol and beta-carotene in the placebo group. The lack of an increase in beta-carotene in the men may have been due to their higher retinol requirements. Lastly, the increase in retinol may have been responsible for the increase in tocopherol and zinc in plasma since these nutrients increased in those whose plasma retinol increased, irrespective of treatment.