The cellular and reaginic immune responses to ragweed were examined in six strains of inbred rats. Three differentiable patterns of primary reaginic responses were observed: (a) Lewis rats--good uniform responders--produced high titers of IgE antibody for a short (less than 2 weeks) time; (b) Brown Norway rats--good uniform responders--produced high titers of IgE antibody for a long time (more than 1 month), and (c) intermediate responders--Buffalo, ACI, Fischer 344 and Wistar-Furth rats--produced poor-to-moderate titers of IgE for an intermediate time. There were no secondary reaginic immune responses to ragweed AgE in these six strains of rats. Furthermore, extensive study of Lewis rats indicated that IgE antibody production in primary immunization could be suppressed by passive transfer of either AgE-primed spleen lymphocytes, AgE-primed blood lymphocytes, or Con-A stimulated lymphocytes. Cellular responses to AgE and PHA stimulation were suppressed in rats receiving AgE-primed lymphocytes or Con-A stimulated lymphocytes.