Leukaemic blast cells from 12 patients with acute leukaemia were examined in order to study their capacity to produce anion superoxide in the absence and in the presence of phorbol-myristate acetate (PMA). Blast cells are able to mount an oxidative respiratory burst upon challenge with PMA, as demonstrated by superoxide release. The production of anion superoxide by blasts committed to monocytic differentiation might be an additional factor contributing to the tissue damage observed in leukaemic patients.