In the present study, FcεR+ and FcεR–– T cells were isolated from patients with the hyper-IgE syndrome and maintained in long-term cultures with interleukin 2. Supernatants from the FcεR+ but not from the FcεR–– T cell lines enhanced IgE but not IgG synthesis in B cells derived from patients with allergic rhinitis. There was, however, no induction of IgE synthesis in B cells from nonatopic donors. The IgE-potentiating factors bound to IgE-Sepharose but not to IgG-Sepharose. The target B cells for these IgE binding factors appear to be preactivated IgE-bearing B cells.