Aggregation phenomena in aqueous solutions of purified human tracheobronchial mucin have been studied by theological methods, steady-state fluorescence, quasielastic light scattering, and spin probe techniques. At temperatures below 30 degreesC and concentrations above 15 mg/mL and in the absence of chaotropic agents, mucin solutions are viscoelastic gels. A gel-sol transition is observed at temperatures above 30 degreesC that is manifested by the diminishing storage modulus and a loss tangent above unity throughout the studied frequency range of the oscillatory shear. No decline in the mucin molecular weight is observed by size-exclusion chromatography above 30 degreesC in the absence of redox agents or proteolytic enzymes. Aggregation of hydrophobic protein segments of the mucin chains at 37 degreesC is indicated by QELS experiments. The decreasing polarity of the microenvironment of pyrene solubilized into mucin solutions at temperatures above 30 degreesC, concomitant with the gel-sol transition, shows the hydrophobicity of the formed aggregates. ESR spectra of the fatty acid spin probe, 16-doxyistearic acid indicate that the aggregate-aqueous interface becomes more developed at elevated temperatures.