Twenty-two children with measles were studied at the stage of the rash and 6 weeks later, and results compared with matched controls. The total lymphocyte count and lymphocyte subpopulations with T- and B-cell markers and those with absence of both markers (null cells) were significantly below control levels in the acute phase. At 6 weeks the B-cell and null-cell counts were still significantly diminished. The function of T cells assessed by 14C uptake of phytohaemagglutinin-stimulated lymphocytes and the delayed skin hypersensitivity reaction to dinitrochlorobenzene was impaired during the acute stage, and this persisted for 6 weeks. Over the 6 weeks of study there was a small but significant rise in serum IgG, IgM and complement factors.