Fifty-two-week oral toxicity of a new glucan (Glucanil, Gluimmun) extracted from Candida albicans ATCC 20955 was investigated in rats. The glucan was orally administered in dose levels up to 200 mg/kg/d and was well tolerated. No deviation from normality was observed in mortality, physical appearance and general behaviour of the treated animals. Food and water consumption and body weight gain of glucan-fed groups did not differ from those of control animals. In these groups no alteration of the weight of the main organs was also observed. Hematology, blood chemistry, urinalysis and autopsy findings were within normal ranges in every group of rats treated. No sex difference was noted. In the 200 mg/kg group soft stools or diarrhoea and cecal enlargement with variable hyperplasia of the colon mucosa were observed. These symptoms are typical of exposure to substances which are absorbed incompletely in the small intestine and subjected to microbial metabolism in the cecum and colon. Diarrhoea, cecal enlargement and mucosal hyperplasia are reversible. The no-effect dose level was estimated to be 100 mg/kg/d under these conditions.