Successive samples of serum of rabbits infected with S. japonicum, and of uninfected controls, were fractionated by paper electrophoresis and changes were observed for a period of seven months. In the early stages of the disease, before deposition of eggs, no notable changes in the serum-protein fractions were observed. However, beginning with early oviposition a marked increase in total proteins was observed. At this time there were also a sudden relative decrease in albumin and increase in gamma-globulin fractions which remained essentially unchanged up to the end of the experiment, 6 months following infection. Fractionations of sera from persons with proven schistosome infection showed significant increases in total protein and in the relative proportions of alpha-2, beta and gamma-globulin fractions, with a corresponding decrease in the relative proportions of albumin.