An in vitro flow cytometric model has been developed to evaluate the effects of antiallergic drugs such as cetirizine (CTZ) on the expression of surface molecules on primary cultured normal cells. Quantitative analysis demonstrated that HLA class I and ICAM-1/CD54 molecules are present on both epithelial and stromal cells, and that their expression is strongly enhanced by treatment with interferon-gamma (IFN-gamma). Nevertheless, the IFN-gamma-mediated upregulation of ICAM-1/CD54 was inhibited by treatment with CTZ, demonstrating a direct effect on both cell types. This finding is particularly interesting because ICAM-1/CD54 is the main rhinovirus receptor, and rhinoviruses are the principal cause of asthma exacerbation in children. Thus, according to data derived from this in vitro model, CTZ should have an important role in the reduction of infectious exacerbation of asthma in atopic patients.