Abstract The Rocky Mountain Laboratory (RML) strain of albino mouse responds to the injection of hens' egg albumin (EA) and soluble extracts of Bordetella pertussis with the production of two homocytotropic antibodies. One of these antibodies (referred to as 2-hr PCA antibody) is a 7S immunoglobulin which does not persist in the skin of mice for a long time and is heat stable and resistant to reduction and alkylation. The other antibody (referred to as 72-hr PCA antibody) is similar to the reaginic antibody of allergic persons, since it is inactivated by heating undiluted sera at 56 C for 30 min and by reduction and alkylation; it remains in the skin for a long time and is not a 7S or 19S immunoglobulin but probably a new type similar to the IgE immunoglobulin of allergic persons. B. pertussis alkaline saline extract (SE) was the best adjuvant for the production of the 72-hr PCA antibody, while Freund's adjuvant was superior in stimulating the 2-hr PCA antibody. Endotoxins from Salmonella minnesota or from B. pertussis were not as effective in stimulating the 72-hr PCA antibody as was the SE. The production of 72-hr PCA antibody in RML mice persisted for at least 60 days after a single immunization with EA and SE, while the sera of mice immunized with EA in Freund's incomplete adjuvant failed to give 72-hr PCA 40 days after immunization. The 72-hr PCA antibody was found to be resistant to heating at 56°C for 30 min. if the serum was diluted in saline prior to heating, which indicated that this antibody is inactivated by a mechanism other than heat denaturation.