Total protein, hemoglobin, glucose, and urea concentrations in hemolymph of Biomphalaria glabrata were studied during a 12-week infection of the snails with Schistosoma mansoni. The results suggest an equilibrium between host and parasite metabolism, which is reached after about 6 weeks of infection. During the period studied, the protein spectrum of infected snails showed no difference in quality compared with that of noninfected snails; all fractions were present, and no additional fractions appeared. Only the lipoprotein fraction increased fourfold in the 1st week of infection, decreasing during the 2nd week to concentrations that were also found in noninfected individuals. No further significant alterations could be detected until week 12 postinfection (PI).