The course of disease due to Schistosoma japonicum in C57BL/6J mice was examined. Animals infected with 25 cercariae of the Philippine strain of S. japonicum for 9 wk develop a peak portal hypertension of 13 +/- 0.6 cm H2O and mean hepatic granuloma area of 459 X 10(3) micrometers 2. By 30 wk, disease manifestations were reduced; mean portal pressure was 7 +/- 0.5 cm H2O and mean hepatic granuloma area was 12 X 10(3) micrometers 2. Adoptive transfer of serum but not 1 X 10(8) spleen or 4 X 10(7) lymph node cells from chronically infected mice resulted in lowering portal pressure and decreasing granuloma size in recipients. Granuloma formation in the sensitized lung model was also suppressed by serum from chronically infected mice. The immunosuppressive component of this serum was localized in the IgG1 fraction.