Low levels of zinc in serum and high in blood clots were found in 12 patients with Down's syndrome (DS). They also had an immune deficiency characterized by depressed neutrophil chemotaxis, skin hypersensitivity and lymphocyte responsiveness to PHA in vitro. The patients received zinc sulphate, corresponding to 135 mg Zn++ daily. After treatment for 2 months' serum zinc levels had increased, neutrophil chemotactic responsiveness was normal in 11 of the 12 patients and there were improved lymphocyte responses to high doses of PHA in vitro. Delayed skin reactivity to DNCB was also improved in 10 of 11 patients after zinc therapy. Zinc deficiency may in part explain the increased susceptibility to infection in DS.