1. The viscosity of sputum in patients with chronic bronchitis has been measured with the Weissenberg rheogoniometer, a cone and plate system, using oscillatory movement at low shear rates. 2. A characteristic pattern is described not previously reported for sputum or any other biological fluid. 3. Sputum viscosity decreased steadily with increasing shear rate; over a narrow range of shear rates a ‘notched plateau’ was shown. The destruction of this plateau by higher frequencies of oscillation was attributed to breakdown or rearrangement of the gel structure. 4. The effects of time on viscosity measurement, presumably caused by drying, are also described.