Four distinct antigenic determinant groups have been found to be associated with β1C-globulin. Antigen A is present on β1A and antigen D is on a second breakdown product of β1C, designated α2D. Both are also present on β1C. Another antigen is also present on α2D which has been designated Dd but this is not present on β1C. A fourth antigen of β1C, designated B, is completely lost from the serum as β1C breaks down with aging. Antiserum containing antibody only to this group forms a β1C are with fresh serum but does not react with aged serum. α2D loses most of its antigenicity on heating serum for 1 hr at 56°. β1A is unaffected by this treatment. The breakdown of β1C occurring on reaction of fresh serum with zymosan or immune precipitates resembles in all respects that occurring with aging. Hydrazine or ammonia treatment, however, diminishes the mobility of α2D. β1G, produced by reaction of isolated β1C with EAC′1,4,2a cells, has A, D and Dd antigens but lacks the B antigen. Its mobility is similar to that of β1A. The studies suggest that β1C undergoes marked unfolding and configurational changes as it breaks down. A close relationship may be assumed between the B antigen of β1C and the activity of the molecule in immune hemolytic reactions.