Myelosuppression concomitant with increased numbers of spleen CFU-GM was observed in tumour-bearing mice. Treatment of these animals with Euphorbia tirucalli L. (ET) (125, 250 and 500 mg/kg) stimulated marrow myelopoiesis and reduced spleen colony formation, with no differences observed between the effects of the three doses. The changes produced by the tumour in total and differential marrow cell counts were restored by the treatment with ET. Prostaglandin E2 (PGE2) levels, which were dramatically increased in tumour bearers, was also abrogated by the treatment with the plant extract. Euphorbia tirucalli L. significantly enhanced survival and concurrently reduced tumour growth in the peritoneal cavity. We propose that the modulatory effect of Euphorbia tirucalli L on myelopoietic response and on the levels of PGE2 may be related to its antitumour activity as a possible mechanism for the regulation of granulocyte and macrophage production and expression of functional activities. (c) 2005 Elsevier B.V. All rights reserved.62294299