RÉSUMÉ
1. In schistosomal infection, the hyperergic acute phase of the disease evolves progressively into the chronic one, with establishment of a relative equilibrium between the parasites and the corresponding host responses. This down-regulation of host reactivity is considered to be under the control of T-lymphocyte circuits. 2. In the present study, we investigated lymphocyte populations in spleens of normal mice and the kinetics of the B-cell number increase in mice in the acute, chronic and late chronic phases of schistosomal infection, and we monitored their proliferation and activity in antibody isotype secretion. 3. We observed polyclonal B-cell activation and modulation of Ig isotype production, compatible with the alternate predominance of TH2 and TH1 lymphocyte subsets, in the acute and the chronic phases of the disease, respectively.