In conditions associated with stimulation of cellular immunity and enhanced macrophage activity, for example, in viral infections, neopterin is elevated. Acute exacerbations of bronchial asthma--in particular in the case of intrinsic asthma--are frequently precipitated by viral infections of the upper airways. In both extrinsic and intrinsic asthma, neopterin is normal in the stable phases. In the exacerbation phase with signs of infection of the upper airways, however, neopterin in significantly elevated both in the serum and in the urine. In contrast, during and following positive inhalative provocation testing with histamine, allergens and aspirin, no increase in neopterin is to be observed.