Confluent spread of mature nondividing mast cells is obtained after 1 month's growth of lymph node cells taken from mice immunized with horse serum and plated on embryonic fibroblast monolayers. The degranulation capacity of these mast cells and histamine release stimulated by monoclonal IgE antibodies and their antigens were studied. The mast cells were first incubated with either anti-2,4-dinitrophenyl (DNP) IgE or with anti-ovalbumin (OVA) IgE and then with the other IgE to study the ability of one IgE specificity to saturate the receptors and block the ability of the other IgE to bind and evoke degranulation. Saturation of the receptors and maximal histamine release (86-92%) were obtained within 2-3 h incubation with excess IgE (1-10 micrograms/ml). No histamine was released after incubation for 3-4 h with the other IgE (0.38-4.0% histamine release). Seventeen days after the excess, unbound, saturating IgE anti-DNP was washed away, 75% of the histamine was still released after antigen DNP-bovine serum albumin was added. However, these mast cells were still effectively blocked from sensitization with IgE anti-OVA (1.71% histamine release). The blocking could be broken if the second IgE molecules were allowed to stay longer than 4 h in culture. From 10 h onwards, the degranulation capacity steadily increased (from 59.6% histamine release after 10 h to 79.5% after 42 h). In studies with 125I-labeled IgE, there was a direct correlation between the rate of binding of the second IgE to the mast cells (from 1.7% binding after 3 h to 75% after 48 h) and the increase in degranulation rate with the second antigen (from 2.40% histamine release to 65.4%). In contrast, only slight binding of the 125I-labeled IgE of the saturating specificity occurred (4.2% after 3 h to 12.7% binding after 48 h). Incubation of mast cells, previously saturated with 125I-labeled IgE, with cold IgE of either specificity did not proportionally reduce the cell-bound label. This suggests that no substitution of IgE molecules on the receptors occurred. When the mast cells saturated with anti-OVA were incubated with IgE anti-DNP together with tunicamycin, the development of the degranulation capacity by DNP-bovine serum albumin was inhibited. The results suggest that IgE molecules of the other specificity stimulated the appearance of new receptors on the mast cell surface.