Studies of granulomatous hypersensitivity to Schistosoma japonicum eggs were performed at various time periods up to 20 wk after the induction of light infections in mice. Cell populations which were determined in granulomas isolated from the livers revealed a maximum in the total number of cells at 6 wk with a decline of 36% by 20 wk. Large mononuclear cells were predominant at all time periods, with eosinophils being the second most common cell. Measurements of granuloma diameters around single viable eggs in the livers also revealed peak size at 6 wk with a decline of 51% between 16 and 24 wk. Immunodiffusion analysis demonstrated the presence of precipitating antibodies as early as 7 wk after infection. Investigations of lymphocyte blastogenesis revealed a profound depression in response to T-cell mitogens by 8 wk of infection. Studies of footpad swelling to soluble S. japonicum egg antigens revealed massive immediate reactions starting at 6 wk, but no delayed reactivity over a period from 3 to 20 wk. All of these results are related to differences in the biology of S. japonicum in comparison with S. mansoni with respect to the earlier onset of egg production, the much larger numbers of eggs produced, and the possibility of differences in the antigens emitted by the eggs.