IgE antibodies against trinitrophenylated ovalbumin (TNP-OVA) were raised in Brown Norway rats by a highly reproducible immunization procedure. With these rats, the kinetics of the IgE response was studied, both for free circulating and cell-bound IgE. Cell-bound IgE was dissociated from peritoneal cell suspensions at acid pH; TNP-specific and total IgE in serum and in eluates of peritoneal cells were determined by radioimmunoassay. Both for total and TNP-specific IgE, a significant correlation was found between the serum IgE level and the amount of IgE molecules per mast cell. One day before TNP hapten could induce histamine release from mast cells, TNP-specific IgE appeared in the circulation and on the cells. These results suggest that also in active sensitization, there is a lag time for sensitization of mast cells.