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rity model hypothesizes that cytoskeleton-based microtu-bules (MTs) carry compression as they balance a portion of cell contractile stress. To test this hypothesis, we used trac-tion force microscopy to measure traction at the interface of adhering human airway smooth muscle cells and a flexible polyacrylamide gel substrate. The prediction is that if MTs balance a portion of contractile stress, then, upon their dis-ruption, the portion of stress balanced by MTs would shift to the substrate, thereby causing an increase in traction. Mea-surements were done first in maximally activated cells (10 M histamine) and then again after MTs had been disrupted (1 M colchicine). We found that after disruption of MTs, traction increased on average by 13%. Because in activated cells colchicine induced neither an increase in intracellular Ca2 nor an increase in myosin light chain phosphorylatio