In five children with measles who subsequently died and in one with measles in whom chronic bronchopneumonia developed (group A), immunosuppression was more pronounced during the acute rash (i.e., 3-20 days before death) than in six children with measles who recovered (group B). The absolute total lymphocyte-count (T and B cells) was significantly lower in group A. Mean serum-C3 was also lower in group A than in group B. There were no significant differences between the two groups for other complement factors or for serum-immunoglobulins. The mean phytohaemagglutinin stimulation index (S.I.) for lymphocytes from patients in group A resembled that in group B, although the S.I. in both groups was significantly lower than that in healthy controls. S.I.S were lowest in two patients who died. Counts of total white cells, neutrophils, null cells, and those with both B and T cell markers were not significantly different in groups A and B. The total lymphocyte-count (mean 2117 +/- S.E.M. 375 cells/mm3) in a further nineteen patients with measles who had died, studied retrospectively, was significantly lower than that (4487 +/- 540 cells/mm3) in twenty-seven patients with measles who recovered.