We have investigated the effect of ascorbic acid (vitamin C) supplementation on some parameters of the human immune defence system in a group of 25 healthy, male university students. The subjects ingested 1 g ascorbic acid per day for a period of 75 days. Serum levels of IgA, IgG, IgM, C-3 complement component, cortisol and transcortin were measured before and after the ascorbic acid course. Corresponding measurements were performed on a control group of 20 healthy, male university students receiving no extra-dietary vitamin C. Our results showed that ascorbic acid supplementation caused a statistically significant increase in the serum levels of IgA, IgM and C-3 complement. Our study does not permit of conclusions regarding the mechanisms of action of ascorbic acid.