We have observed differences in apparent sensitivity between rheological techniques used to monitor the effectiveness of mucolytic treatments. For example, low concentration of the DNA-lytic protein rhdNase (100 nM) produced a mean 64% decreased (p<0.001) in spinnability in CF sputum samples compared to saline control (Eur. Respir. J. 7, 1994, 279s), while the same treatment produced a nonsignificant fall (mean 15%) in viscoelasticity (G * ) as determined by magnetic rheometry. Similar discrepancies in apparent sensitivity were made for gelsolin (100 nM), which limits F-actin network formation in sputum, and Nacystelyn (10 μM), a thiol reducing agent. However, for treatment of CF sputum with hypertonic saline (0.5 M final conc.), the viscoelasticity reduction was the same magnitude or greater than the reduction in spinnability; this was also the case for the rheological degradation produced by high frequency oscillation (Biorheol. 31, 1994, 511). DNase, gelsolin and Nacystelyn all act by molecular weight reduction of network polymers. The main reason why spinnability by filancemeter is a more sensitive indicator for this type of mucolysis (i.e. mol. wt. reduction) than viscoelasticity by magnetic rheometer is likely related to the molecular weight dependence of the two techniques. Viscosity (and probably viscoelasticity) classically exhibits a 3.4-power dependence on mol. wt. (Ferry, J. D.,Viscoelastic Properties of Polymers ). The power dependency for spinnability is less certain, but to the extent that it is related to the first normal stress difference, its power law dependency should be much higher, theoretically 6.8 power. This extra power dependency would mean that a mere 10% reduction in mol. wt. would reduce spinnability to (0.9)EE6.8 or 49% of control, which is comparable to the reduction in spinnability observed for rhDNase treatment. At the same time, viscoelasticity would only decrease to (0.9) E3.4 or 70%. On the logarithmic scale used for viscoelasticity (log G * ), this reduction would only amount to 0.16 log units, which is close to the usual limit of detectability for magnetic rheometry (0.15-0.2 units). With hypertonic saline, it is assumed that there is no cleavage of intra-molecular bonds and no reduction in chain length; however, shielding of fixed charges should produce substantial reductions in intermolecular interaction and macromolecular conformation, both of which would reduce the degree of entanglement coupling. For this type of alteration, viscoelasticity and spinnability changes should become comparable, in agreement with observation.