To assess human mast-cell (MC) behavior after repetitive activation, we cocultured human foreskin MC (SMC) with human foreskin fibroblasts (F). Under these conditions, we have previously demonstrated that SMC keep their viability and functional activity for up to 8 days. SMC were presensitized with atopic serum and repeatedly activated by consecutively increasing concentrations of anti-IgE antibodies (alpha-IgE, 0.0002-0.1%). This treatment, which mimics the "rush desensitization" procedure, led to complete SMC unresponsiveness to activation by alpha-IgE at optimal concentrations, as evaluated by histamine release. However, presensitization of SMC with IgE antibodies before exposure to alpha-IgE restored their sensitivity to this stimulus. These data indicate that desensitization was probably due to lack of membrane-bound IgE rather than to downregulation of intracellular mechanisms. In fact, SMC challenged by an optimal concentration of alpha-IgE could release histamine upon a second activation by 2 h after the first activation, if the cells had been presensitized before the second challenge. SMC incubation with increasing concentrations of compound 48/80 (0.2-10 micrograms/ml) led to MC unresponsiveness to an optimal concentration of this stimulus. Furthermore, SMC activated by an optimal concentration of compound 48/80 and rechallenged with the same agent were insensitive to the second activation for at least 24 h. In summary, we have shown that it is possible to induce "desensitization" in SMC to both IgE-dependent and IgE-independent stimuli by incubating the cultures with consecutively increasing concentrations of the activator. SMC can release histamine when reactivated with alpha-IgE antibodies after presensitization by 2 h after the first challenge, while they reacquire their susceptibility to reactivation with compound 48/80 in only 2-3 days.