1. Ciliary activity is significantly influenced by chemical and physical properties of the liquid medium in which the cilia beat.

2. We studied the effect of changes in pH, ionic strength and viscosity on the ciliary beat frequency (CBF) of explants of human respiratory mucosa.

3. Optimal CBF was elicited at pH 7.0-9.0, with a marked reduction of CBF outside these limits. The CBF was well preserved at NaCl concentrations between 5 g/l (80 mmol/l) and 12 g/l (200 mmol/l), but there was rapid loss at concentrations below 0.5 gA (10 mmol/l). The cilia beat best at viscosities below 1.0 centipoises (1 mN s m−2). Increase of the viscosity gradually decreases CBF with a significant drop at viscosities above 87 millipoises.

4. It is concluded that the above limits may fairly accurately indicate the actual physical characteristics of the periciliary environment (‘sol layer’) in vivo.

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