The antibody activity responsible for the immunologic release of SRS-A in the rat was present in rat serum obtained 1 week after immunization with DNP-BGG in complete Freund's adjuvant. Peak activity was present at weeks 2 and 3 and declined to negligible levels by week 10. The antibody activity was associated with the IgGa class of rat immunoglobulins and was stable to heating at 56°C for 4 hr. Relatively small amounts of nonspecific IgG interfered with the capacity of IgGa antibody to prepare rat tissue for SRS-A release.