Monodisperse aerosols can be used to study fundamental questions in lung physiology. Monodisperse aerosols containing a range of chemical material, including pharmacologically active substances, were produced by using a modified Sinclair-LaMer condensation generator. Instead of the usual hot-wire technique, an ultrasonic nebulizer was used to suspend nuclei in the gas phase as an aqueous polydiperse aerosol. This technique allowed the generation of monodisperse oil (sebacate) aerosols containing nuclei of NaCl, histamine, methacholine, antigen, iron oxide, methylene blue, uranine, radioactive technetium pertechnetate and technetium-labeled human serum albumin. Quantitative agonist activity was demonstrated for each active nucleus. Particle sizes ranged from 0.65-3 .mu.m, with a geometric SD of 1.1-1.2. This system can produce a stable, monodisperse, biologically active radiolabeled aerosol for a period of several hours.