One hundred and ninety-one adults with acute myelogenous leukaemia were treated with combination chemotherapy consisting of daunorubicin and cytosine arabinoside (Barts III). Sixty-three patients achieved remission and were admitted to one of 3 trials of active immunotherapy: immunotherapy alone, immunotherapy and maintenance chemotherapy or neither of these. All patients had weekly clinical and blood examination and monthly marrow examination. Reinduction chemotherapy was given as soon as relapse was diagnosed in the marrow. The most striking observation was that immunotherapy was associated with easy and repeated reinduction of remission and marked prolongation of survival after first relapse when compared with immunotherapy plus chemotherapy. The possible reasons for this and the value of immunotherapy are discussed in relation to the third trial still in progress which includes 2 maintenance arms, immunotherapy alone and surveillance only.