The effect of a semi-synthetic cephalosporin, Cefadroxil, on antigen-induced bronchial hyperresponsiveness and eosinophil accumulation in lungs from sensitized guinea pigs was investigated and compared to the effects of Cetirizine and Ketotifen. When aerosol-sensitized guinea pigs were pretreated 1 h before the antigen challenge with Cefadroxil (100 mg/kg i.p.) a partial but significant inhibition of the bronchial hyperresponsiveness to aerosolized acetylcholine chloride was observed. Furthermore, the treatment of guinea pigs (115 mg/kg, per os) 24 and 1 h before ovalbumin challenge also significantly reduced bronchial hyperresponsiveness. In contrast, no significant inhibition was noted when the guinea pigs were treated by a single dose of Cefadroxil (115 mg/kg per os) 1 h before challenge. Pretreatment of the guinea pigs with Cetirizine (1 mg/kg per os) or Ketotifen (0.1 mg/kg per os) completely inhibited the antigen-induced bronchial hyperresponsiveness. Cefadroxil (100 mg/kg i.p.) slightly inhibited the accumulation of eosinophils in the peribronchial area induced by antigen challenge. In contrast, no significant reduction was noted when the guinea pigs were treated per os with Cefadroxil (115 mg/kg), Cetirizine (1 or 10 mg/kg) or Ketotifen (0.1 mg/kg). These results show that Cefadroxil is effective in reducing antigen-induced bronchial hyperresponsiveness, an effect independent of a reduction in the pulmonary inflammation, namely eosinophil accumulation in lung.