Administration of the colour additive Ammonia Caramel Colour (Caramel Colour III) to rats has been associated with decreased lymphocyte counts, specifically in rats fed a diet low in vitamin B6. This effect is rapidly reversible and is caused by an imidazole derivative (THI) in Caramel Colour III. In the present paper, the conduct of a human study with Caramel Colour III is outlined and the results of blood lymphocyte counts are presented. No decrease in the number of blood lymphocytes occurred in marginally vitamin B6-deficient humans who consumed Caramel Colour III at the acceptable daily intake level (200 mg/kg body weight/day) for 7 days. These data are discussed in relation to the effects of Caramel Colour III and THI on blood lymphocyte numbers in rats.