In addition to its more widely recognized role in promoting IgE synthesis, we speculate that interleukin-4 (IL-4) may modulate both allergic- and nonallergic-type inflammatory processes in the airway mucosa. We examined in vivo the effect of IL-4 on granulocyte and cytokine homeostasis in the nasal airways of nonallergic volunteers. Ten (N = 10) healthy subjects received nasal IL-4 (10 microg) or saline (0.9%) challenges on separate occasions. Nasal lavage was obtained before and 24 h after nasal challenges. We report that IL-4 induced a significant increase in IL-6 and produced elevated levels of eosinophils and neutrophils compared to saline. These data demonstrate that IL-4 can modulate both allergic- and nonallergic-type inflammatory responses in the nasal airways of nonallergic individuals.