Effects of lithium carbonate on peripheral white blood cell and granulocyte counts were investigated in children treated for acute lymphoblastic leukaemia and non-Hodgkin malignant lymphoma. Li2CO3 given orally for two weeks in a single daily dose of 700 mg/m2 caused a significant and lasting increase in the peripheral WBC and granulocyte counts and increased the granulocyte ratio during induction of remission and maintenance cytotoxic therapy. Haematologic actions and the long-term effect of lithium carbonate are discussed.