Twenty eight adult patients of both sexes with chronic bronchitis participated in an open study to determine the effect of intranasal treatment with IRS-19, an immunomodulating agent, on the number of polymorphonuclear leukocytes (PMNL), H2O2 concentration and myeloperoxidase (MPO) activity in nasal washings. The number of PMNL recovered from nasal spaces increased from 4460 +/- 3960 to 10,490 +/- 10,950 cells/ml (p < 0.02) after two month administration of IRS-19. It was accompanied by 2.6- and 1.4-fold increase (p < 0.001) in MPO activity and H2O2 concentration, respectively. However, no correlation was found between increments in these three variables. Since PMNL and MPO-H2O2-Cl- system are involved in the first line of defense against invading pathogens it is suggested that above mentioned changes may represent one among mechanisms leading to enhancement of antibacterial defence in the airways in response to treatment with IRS-19.