Allergic asthma is a chronic inflammatory disorder of the airways characterized by biphasic airway obstruction and airway hyperresponsiveness. In this study, we attempted to elucidate the contribution of the complement C3a to these asthmatic symptoms. BALB/c mice sensitized by i.p. injections of OVA plus alum were challenged with OVA intratracheally four times. The fourth challenge caused a biphasic asthmatic response peaking at 10 min and 3-4 h, as well as airway hyperresponsiveness to methacholine. Histological examination revealed increased expression of C3a receptors in the lung on the fourth challenge. Additionally, the C3 level in serum 4 h after the fourth challenge was significantly reduced compared with that before the challenge. When a C3a receptor antagonist, SB290157, was administered i.p. 30 min before the fourth challenge, the late-phase asthmatic response and airway hyperresponsivness induced by the fourth challenge were significantly inhibited, although the early-phase response was not influenced. In bronchoalveolar lavage fluid, neutrophil infiltration 24 h after the fourth challenge was reduced by the treatment. On the other hand, SB290157 suppressed the increased expression of IL-1beta in the lung in this model, and the intratracheal administration of IL-1beta induced airway obstruction, airway hyperresponsiveness, and neutrophil infiltration in normal mice. These results illustrate that C3a is involved in the development of the late asthmatic response and airway hyperresponsiveness. The mechanism leading to the development of these symptoms may correlate with the recruitment of neutrophils and/or the production of IL-1beta induced by C3a.