Mast cells were purified by discontinuous density gradient centrifugation to greater than 90% and subsequently exposed to lactic acid in order to dissociate endogenous IgE. Unfortunately no removal of this IgE was achieved, despite previous reports of lactic acid exposure dissociating IgE from human basophils. Only a minimal level of endogenous IgE was found on the surface of mast cells, however, which allowed investigations of IgE binding to proceed without the initial removal of this endogenous IgE. Conditions for binding of normal rat IgE and myeloma IgE were established while efforts to bind human IgE to purified rat mast cells were unsuccessful. Our results suggest that while purified rat mast cells have the potential to play an important role in the study of the IgE requirement of various stimuli including histamine-releasing factor, this role is limited at present.