The close relation between rat mast cells and rat basophilic leukemia (RBL) cells with regard to the presence of receptors for IgE and Fc gamma led us to generate monoclonal antibodies directed against cell surface antigens. Hybridomas were obtained by the fusion of NS1 mouse myeloma cells with murine spleen and lymph node cells. The culture supernatants were assayed by two ELISA techniques: a) for the production of mouse immunoglobulin in general and b) for antibodies directed against surface antigens of RBL cells. For this purpose RBL cells were attached to polyvinyl chloride microtitre plates. Eight hybrids produced antibodies directed against surface antigens on RBL cells. Hybrids were cloned and characterized with regard to their isotype and light chains. All eight clones secreted IgM with K light chains. Immunofluorescence studies performed with RBL cells revealed that all eight antibodies were able to show a specific fluorescence. Furthermore, four of these eight antibodies also showed a specific fluorescence with purified rat mast cells. These four antibodies were analyzed as to their ability of interacting with the IgE-receptor on RBL cells and purified rat mast cells. They reduced the binding rate of radiolabelled rat IgE to RBL and rat mast cells. A mutual inhibition of the passive cutaneous anaphylaxis (PCA) reaction in the rat by either mixing mouse reaginic serum directed against 2,4-dinitrophenol bovine serum albumin (DNP-BSA) or by mixing monoclonal mouse anti-DNP IgE with the monoclonal mouse anti-cell surface (rat basophilic leukemia, rat mast cell) IgM was determined.(ABSTRACT TRUNCATED AT 250 WORDS)