Inhalational challenge with antigen decreases the function of inhibitory M2 muscarinic autoreceptors on parasympathetic nerves in the lung, increasing the release of acetylcholine from the vagus nerves and potentiating vagally induced bronchoconstriction. It is possible that eosinophils cause M2 receptor dysfunction, perhaps by releasing positively charged proteins that are M2 receptor antagonists. Because of the probable role of interleukin-5 in initiating and maintaining the eosinophil infiltration, we tested the function of neuronal M2 receptors in antigen-challenged guinea pigs after pretreatment with a monoclonal antibody to interleukin-5 (TRFK-5). Ovalbumin was given intraperitoneally to sensitize the animals. Three weeks later, the animals were injected intraperitoneally with either TRFK-5 (240 micrograms/kg i.p.) or saline. Beginning three days later, they were challenged with an ovalbumin aerosol for 5 min on each of four consecutive days. M2 receptor function was tested 24 h after the last antigen challenge. Electrical stimulation of both vagi caused bronchoconstriction and bradycardia. In control animals, pilocarpine attenuated, and gallamine potentiated, vagally induced bronchoconstriction by stimulating and blocking neuronal M2 muscarinic receptors, respectively. In challenged animals that did not receive TRFK-5, these effects were markedly reduced, confirming M2 receptor dysfunction. In TRFK-5-treated guinea pigs, the effects of both pilocarpine and gallamine were the same as those in control animals, demonstrating normal M2 receptor function. Pretreatment with TRFK-5 selectively inhibited the migration of eosinophils into the lungs as measured by lung lavage. Thus the function of M2 muscarinic receptors in antigen-challenged guinea pigs can be protected by inhibiting eosinophil influx into the lungs.