Numerous studies have suggested an important role for the Th2 cytokines interleukin (IL)-13 and IL-4 in the development of allergic asthma. We tested the hypothesis that IL-13 and IL-4 have direct effects on cultured airway smooth muscle cells (HASM). Using RT-PCR, we showed that HASM cells express transcripts for IL-4 α , IL-13R α I, and IL-13R α II, but not for the common IL-2R γ chain. We then analyzed the capacity of the two cytokines to activate signaling pathways in HASM cells. Both IL-13 and IL-4 caused STAT-6 phosphorylation, but the time course was different between the two cytokines, with peak effects occurring 15 min after addition of IL-4 and 1 h after addition of IL-13. Effects on signaling were observed at cytokine concentrations as low as 0.3 ng/ml. IL-4 and IL-13 also caused phosphorylation of ERK MAP kinase. As suggested by the signaling studies, the biological responses of the two cytokines were also different. We used magnetic twisting cytometry to measure cell stiffness of HASM cells and tested the capacity of IL-4 and IL-13 to interfere with the reductions in cell stiffness induced by the β -agonist isoproterenol (ISO). IL-13 (50 ng/ml for 24 h), but not IL-4, significantly reduced β -adrenergic responsiveness of HASM cells, and the MEK inhibitor U0126 significantly reduced the effects of IL-13 on ISO-induced changes in cell stiffness. We propose that these direct effect of IL-13 on HASM cells may contribute at least in part to the airway narrowing observed in patients with asthma.