Interleukin-5-producing CV-1 cells were encapsulated in alginate and injected i.p. in guinea pigs (4 x 106/animal). These cells produced approximately 8 ng interleukin-5 per 4 x 106 cells per day. Airway hyperresponsiveness to histamine in vivo was observed 3 and 7 days after administration. The increase in lung resistance after intravenous administration of histamine to guinea pigs was significantly potentiated, by approximately 70 to 90% in interleukin-5-treated animals. In animals treated with antibody to interleukin-5, the administration of interleukin-5-producing CV-1 cells did not induce hyperresponsiveness. The percentage of eosinophils in broncho-alveolar lavage fluid was increased by 100% at 7 days but not a 3 days after administration of interleukin-5-producing CV-1 cells. Antibody to interleukin-5 prevented the broncho-alveolar lavage eosunophilia at 7 days after interleukin-5 administration. It can be concluded that interleukin-5 induces broncho-alveolar lavage eosinophilia and airway hyperresponsiveness and that these phenomena do not occur simultaneously. These data suggest a role for interleukin-5 in the development of airway hyperresponsiveness in bronchial asthma.