Summary: Highly specific antibody to mouse β1C-globulin has been produced in rabbits by injecting incomplete Freund's adjuvant containing mouse complement adsorbed onto zymosan. The production of this antisera enabled analysis of the interaction of mouse β1C-globulin with EAC′1,4,2a cells, antigen-antibody precipitates and zymosan. The data revealed that the immediate reaction product upon interaction of β1C-globulin in these in vitro immune systems was a rapidly migrating β-α2-globulin, called β1C-globulin, and that this product was then transferred into an inactive, more slowly migrating component, β1D-globulin. Ultracentrifugal analysis showed β1C-globulin to have an s-rate of 9.5 S, β1D-globulin to have an s-rate of 6.5 S and β1G-globulin to have an intermediate s-rate. A close similarity is shown to exist between human β1C-, β1G- and β1A-globulin and their mouse homologues.