Natural killer (NK) cell activity and related markers were analysed in childhood acute lymphoblastic leukaemia (ALL). Children with untreated ALL, children with active disease, and children in remission for less than 1 month and undergoing induction therapy had significantly lower NK cell activity in peripheral blood than the control group (P less than 0.05, P = 0.0005, and P less than 0.0025). Patients in remission for 1-3 months and undergoing consolidation chemotherapy had normal NK activity (P greater than 0.05). Children in complete remission for more than 3 months and undergoing maintenance therapy also had a normal NK activity in their peripheral blood (P greater than 0.05). However, their bone marrow cells showed an increased NK cell activity (P less than 0.0005). Cells positive for the Leu-7 marker were reduced in the peripheral blood from untreated children (P less than 0.025) and children in remission for less than 1 month (P = 0.025). The percentage of cells from peripheral blood expressing the marker Leu-11b (CD 16) did not differ significantly from that of the controls (P greater than 0.05). However, children in complete remission for more than 3 months had a higher number of bone marrow cells expressing the Leu-7 (P = 0.005) and the Leu-11b (CD 16) markers (P = 0.05) than controls. Stimulation of mononuclear cell suspensions with recombinant alpha interferon and recombinant interleukin 2 were shown to cause a normalization of the NK cell activity in peripheral blood and bone marrow.