Lymphocyte subpopulations were studied in 28 children with acute lymphoblastic leukaemia at different stages of the disease. T and B lymphocytes, present in low percentages at diagnosis, increased towards normal during remission in most patients. All patients off therapy had normal values of T and B lymphocytes. K cell activity, which was increased during remission in several patients, was normal in all patients off therapy. Further prospective studies will establish whether determination of K cell activity is an indicator of the stage of the disease.