The antischistosomal property of a short course of cyclosporin A (CsA), administered at the time of infection was demonstrated in the rat. In comparison with vehicle-treated controls, no schistosomes were recovered from CsA-treated animals, 18-19 days post infection. Flow cytometric analysis of blood and spleen lymphocyte populations from rats protected by CsA revealed no significant changes in functional T lymphocyte subsets, B cells or Ia-positive cells. Moreover, there were no significant differences in eosinophil numbers between the two experimental groups. These data add credence to the view that the unexplained antischistosomal property of cyclosporins is not mediated immunologically.