Protein fractions with dissimilar electrophoretic mobility were identified in Langerhans, islands. In solitory islands, isolated from pancreas of rats with alloxan diabetes as compared with normal rats the protein composition varied distinctly: fractions of "constant" proteins and, specifically, the insulin fraction disappeared completely, total amount of identified fractions was decreased with relative alteration in their mobility. The total protein-synthesizing activity of Langerhans' islands from diabetic animals was 3-4-fold decreased as compared with that of control animals. An effect of "glucose repression" of protein synthesis was observed in isolated Langerhans' islands.