Abstract The serum protein β1C-globulin contains the activity of one of the component parts of the complement factor originally called C′3. β1C-globulin converts easily into β1A-globulin. Immunoelectrophoresis experiments and immune hemolysis experiments were performed to examine the reaction mechanism leading to the formation of β1A-globulin. The effects of various intermediate complexes of the immune hemolysis reaction process were studied. The dependence of β1C-globulin transformation on pre-fixation of C′1, C′4 and C′2 has been studied with egg albumen-anti-egg albumen specific precipitate and sheep erythrocytes sensitized with anti-sheep red cell antibodies. The direct action of hydrazine on β1C-globulin has been demonstrated in kinetic experiments. Experimental results with synthetic peptide esters and with C′1 esterase suggest an enzymatic pathway for β1C-globulin conversion in immunological reactions.