BALB/c mice and BN rats manifesting persistent IgE and IgG responses were examined up to 1 year after immunization. A significant proportion of the ongoing antibody response in these animals survived lethal X-irradiation employing dosages sufficient to deplete B memory cells. The persistent IgE responses in both species were refractory to exogenous isotype-specific suppressor cells taken from tolerant syngeneic animals, which were shown to abrogate primary IgE responses in parallel tests. Employing a novel ELISA-based assay for plaque forming cells, long-lived radioresistant IgE- and IgG-secreting cells were identified in differing ratios in lymph nodes, spleen, and bone marrow of both species. These long-lived cells were shown to arise following maximum antigenic challenge with antigen plus adjuvant, and after repeated low-grade stimulation by antigen alone, including passive inhalation of dilute antigen aerosols.