This study was designed to determine differences in the host response of hamsters to three species of schistosomes using graded natural infections evaluated on the basis of the number of eggs per gram of liver rather than on the number of female worms present at necropsy. For each species, Schistosoma mansoni, S. haematobium and S. japonicum, 220 hamsters were exposed to varying numbers of cercariae. The onset of patency, as determined by detection of schistosome eggs in the feces, was selected as the reference point for scheduling necropsies. This approach was used so that the severity of hepatosplenic schistosomiasis evoked by egg deposition in the liver could be evaluated at comparable stages. Necropsies were conducted at 1, 3, 5, 11 and 19 weeks after eggs were first detected in the feces. Un-infected animals of the same age and lot were also examined. Observations made were: body weight, liver weight, spleen weight, portal pressure, hematocrit, serum protein and electrophoresis, worm counts and eggs per gram of liver. When approximately equal numbers of eggs per gram of liver were present in hamsters with different species, S. japonicum infections consistently exhibited greater alterations in all variables except in the liver weight-to-body weight ratio and in the portal pressure. In general, the host response to S. mansoni infections was similar to that for S. haematobium. This study provides further evidence of an intrinsic qualitative difference in the host response elicited by S. japonicum eggs and indicates that the severity of schistosomiasis japonica cannot be wholly attributed to its greater rate of egg production.