Renewed interest in thalidomide stemmed from the suggestion by several authors that it might prove useful against immune-related diseases. Despite interesting preliminary findings, little is actually known regarding its immunoenhancing properties in vivo. Male and female Swiss mice, aged 6-8 wk, were given 0.5 or 25 mg/kg/day of thalidomide orally for 4 wk. Various immunological parameters were then assessed. When compared to controls, mice exposed to the higher dose presented with a significantly increased thymus weight index, whereas spleen weight index remained unchanged. Anti-sheep erythrocyte plaque-forming cells and hemagglutinin titers were increased by 44% and 29%, respectively. Contact hypersensitivity to picryl chloride was enhanced by 62%. Finally, colloidal carbon clearance was not altered. Interestingly, the lower dose (5 mg/kg/day) exerted no effect whatsoever.