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Naturally occurring infection in humans or experimental infection in mice by Schistosoma mansonilead to a Th1 towards Th2 switch of cytokine response in the chronic late phase of the disease. It has been proposed that an initial Th1 response is successively down-regulated and followed by an egg-antigen driven Th2 response. Here we report the results of a kinetic study on the capacity of spleen cells from experimentally infected Balb/c mice to produce Th1 or Th2 cytokines, following mitogen stimulation, during the phase which precedes the granulomatous response associated to egg-deposition. The main results were identified as an early increase in IL-4 and IL-10 Th2 cytokine production, particularly pronounced for IL-10, only a slight and delayed decrease in IL-2 production and an invariable or actually enhanced capacity to produce IFN-γ. The emergence of Th2 response was associated with only slight and delayed modifications in spleen lymphocyte subsets, mainly represented by a decrease in CD3+ T cells and an increase in B cells. The initial unbalance of the Th1/Th2 response precedes thus the egg-deposition and the chronic phase of the infection, being evident 3 weeks after the challenge with the parasite. This observation could represent a novel finding useful in understanding the complex mechanisms involved in the immunopathogenesis of schistosomiasis.