Rat monoclonal antibodies were constructed by fusion of immunized rat spleen cells with a nonsecreting mouse myeloma cell. Two monoclonal antibodies (6HD5 and HMK-12) were selected for further study. Both reacted with various IgE molecules of different specificities and different allotypes, but did not react with immunoglobulins of other isotypes and with light chains. These antibodies were therefore anti-isotypic (IgE) and not anti-allotypic or anti-idiotypic. It was shown by competition studies that these antibodies recognize different epitopes on the FcR epsilon fragment. A sensitive ELISA for the quantitation of murine IgE was developed with these monoclonal antibodies; the sensitivity was between 2 and 250 ng/ml for detection of serum IgE levels. Good correlation was obtained with protein amounts as determined by enzyme-linked immunosorbent assay (ELISA) and passive cutaneous anaphylaxis (PCA) activities. Both monoclonal antibodies were used to study anaphylactic reactions elicited by IgE antibodies. Both could inhibit PCA reactions and both could elicit reverse PCA reactions.