Hepatic granulomata which were qualitatively and quantitatively similar to those seen in infections established with cercariae were induced by surgical injection of exogenous eggs of Schistosoma mansoni and Schistosoma japonicum via the mesenteric veins of previously unexposed albino mice. Thereafter, their comparative histopathologic studies were made. The maximum mean sizes of granulomata were attained on Day 32 with viable eggs of these parasite species. Although S. mansoni eggs produced significantly larger lesions (368.4 +/- 21.5 microns) than eggs of S. japonicum (205.8 +/- 18.6 microns) at the peak period, the difference in the mean granulomal size showed no correlation with either the time of onset or the severity of pathologic changes produced. Thus, eggs of S. japonicum with smaller granulomata, evoked pathologic changes which were earlier in onset and more severe than those produced by the same quantity of S. mansoni eggs. Since the most obvious variables (the quantity of eggs, the sequence of their arrival in the liver, the timing of observation, and the strain of experimental animals) were controlled, it was concluded that the most severe and fatal effects produced by eggs of S. japonicum, as previously suggested, are largely due to differences in the cytotoxic and antigenic peculiarities of this species.