A mechanistic model for the avascular tumor growth both for in "in vivo" and "in vitro" conditions is presented here. It involves the main cell physiology, like the growth (by substrate uptake), the mitosis, the apoptosis and the necrosis, all with their corresponding kinetics. The resulting population balance was solved through the moments' method. Moreover, the model was combined with the classic equations of pharmacokinetics to consider also the presence of active drugs affecting either the kinetics of cell necrosis or growth. The model was compared with literature tumor growth data both for in vivo and in vitro conditions. (c) 2007 Elsevier Ltd. All rights reserved.