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<0.05,P=0.0005. andP0.05). However, their bone marrow cells showed an increased NK cell activity (P<0.0005). Cells positive for the Leu‐7 marker were reduced in the peripheral blood from untreated children (P0.05). 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 bo