Natural killer cells are postulated to play an important role in host anti-viral defences. We measured natural killer cell activity in 30 individuals with acute measles (73 ± 21 lytic units (LU)/107 cells) and 16 individuals with other infectious diseases (149 ± 95 LU) and found it reduced compared with values for adults (375 ± 70 LU; P < 0.001) or children (300 ± 73 LU, P < 0.01) without infection. Reduced natural killer cell activity was found in measles patients with (84 ± 30 LU) and without (55 ± 18 LU) complications and was present for at least 3 weeks after the onset of the rash. Activity was increased by in vitro exposure of cells to interleukin-2. Depressed natural killer cell activity parallels in time the suppression of other parameters of cell-mediated immunity that occurs during measles.