High frequency of hybridoma lines secreting large amounts of reaginic anti-DNP antibodies were generated by the fusion of splenic lymphocytes enriched by adoptive transfer of immune cells and the NSI plasmacytoma cell line. The murine IgE molecule binds DNP with high affinity and was specifically purified by immunoabsorption. The 190,000 dalton molecule is composed of a lambda-light chain and an epsilon-heavy chain of an apparent m.w. of about 90,000. Absorption studies with anti-class-specific antisera suggest that some of the hybrid clones secrete also a mixed molecule consisting of lambda- and kappa-light chains in addition to the epsilon-heavy chain. The monoclonal reaginic antibody binds to mast cells or rat basophilic leukemia cells and, upon binding of DNP-protein, triggers an anaphylactic reaction.