The aim of present study was to analyze the potential immunomodulatory effects of theophylline. We studied the influence of therapeutic and subtherapeutic drug concentrations on the metabolic human monocytes activity using the chemiluminescence test, that allows to evaluate the production of free oxygen radicals. Subtherapeutic theophylline concentrations of 2.5 and 5 micrograms/ml significantly increased monocytes spontaneous chemiluminescence activity. Moreover, in concentrations 5-20 micrograms/ml theophylline interfered with the process of monocyte activation by zymosan: decreasing O2-total and maximal production as well as affecting duration of the "respiratory burst" reaction. Correlation between theophylline concentration and intensity of its suppressive effect was also observed. Therefore we concluded that theophylline might be useful in the treatment of allergic inflammation characterising bronchial asthma.