T he addition of specific antigen to IgE-sensitized human lung tissue causes the secretion of the mediators histamine and slow-reacting substance of anaphylaxis. The mechanisms by which increased levels of cyclic AMP suppress and increased levels of cyclic GMP enhance this secretory process were studied. Colchicine, an agent which disrupts many secretory reactions by binding to microtubules in their disassembled 6S form, was a relatively ineffective inhibitor of the antigen-induced release of mediators unless lung fragments were incubated at 4°C for 60 min to induce microtubular disassembly. As colchicine appeared to inhibit the immunologic secretion of mediators from human lung tissue most effectively after microtubular disassembly, the capacity of colchicine to modulate the release reaction indicated the state of microtubular assembly; inhibition by colchicine signaled a shift to the colchicine-sensitive 6S subunits whereas failure to inhibit suggested maintenance in the colchicine-resistant polymerized state.Exogenously added 8-Bromo-cyclic GMP prevented low temperature-facilitated colchicine suppression of mediator release suggesting that increased levels of cyclic GMP stabilize polymerized microtubules. Transiently increased cyclic AMP concentrations, either exogenously added as 8-Bromo-cyclic AMP or endogenously produced by isoproterenol, promoted colchicine suppression of mediator release suggesting that microtubular disassembly was produced. Direct measurement of cyclic AMP levels revealed parallel kinetics after isoproterenol stimulation between control and colchicine-treated lung fragments.The requirement for functional microtubules in the release reaction may occur after the antigen IgE-stimulated activation of a serine esterase, energy utilization, and an intracellular calcium requirement. The mechanism by which cyclic nucleotides influence microtubular assembly is postulated to involve the degree of phosphorylation-dephosphorylation of microtubules.