Summary: The antibodies in hyperimmune equine anti-pneumococcal sera which have been described as ultracentrifugally heterogeneous with sedimentation coefficients of 9 S to 15 S were shown to be noncovalently linked aggregates of 6.3 S globulins. The aggregates were dissociated by detergents, urea and elevated salt concentrations but were unaffected by reducing or chelating agents. A goat antiserum produced by immunization with an SXII-horse-anti-SXII specific precipitate reacted with an immunoglobulin which was antigenically distinct from γG-, γM- and T globulins. This horse immunoglobulin appeared to be the reversibly aggregating protein and was the major antibody type in hyperimmune anti-pneumococcal sera. It was antigenically demonstrable in all of 12 horse sera tested, whether fresh or stored, normal or immune. The relative levels of the immunoglobulin seemed to be related to intensity of the immune response rather than to duration of storage of the sera. Its antigenic specificity could not be simulated by proteolysis, reduction and alkylation, or heat denaturation of other horse immunoglobulins.