Brown Norway rats were immunized with trinitrophenylated ovalbumin (TNP-OVA). TNP-specific and total IgE in serum and in acid eluates of pleural and peritoneal mast cells were determined by radioimmunoassay. The IgE load of peritoneal mast cells was significantly higher than the IgE load of pleural mast cells. 8-9 days after immunization, the ratio specific/total IgE for cell-bound IgE was smaller than this ratio for free circulating IgE. The composition of IgE on mast cells that were isolated 8-9 days after immunization was better reflected by the composition of serum IgE on the day before isolation than by the composition of serum IgE at the time of isolation of the mast cells. Therefore, we conclude that (1) IgE on mast cells is not in rapid equilibrium with free circulating IgE, and (2) the composition of IgE on mast cells lags at least 1 day behind that of serum IgE.