In 155 asthmatic children we have studied the relationship between prick test positivity and a set of genetic factors previously found to be associated with bronchial asthma. Among these factors, MN system (p = 0.009) and age at onset of symptoms (p = 0.05) are the most important variables separating prick test negative from prick test positive children. MN and age at onset influence independently prick test positivity pointing to an additive effect of the two variables. M phenotype appears correlated positively with an increased susceptibility to nonallergic asthma in all age groups, whereas N phenotype appears correlated positively with age at onset but in allergic asthma only. The MN system codifies for glycophorin A, a sialoglycoprotein that represents a major ligand for several bacteria and viruses that recognize the N-acetylneuraminic acid present in this protein. The present data suggest that genetic variability in this system might influence bacterial and viral competition and mucosal damage influencing susceptibility to asthmatic reactions in absence of IgE hyperproduction.