The effects of two topical nasal preparations on human nasal ciliary beat frequency in vitro and on nasal mucociliary clearance in vivo were investigated. Betamethasone and betamethasone with neomycin drops were found to be ciliotoxic when ciliated epithelium was exposed to them in vitro, mainly owing to the effects of the preservatives benzalkonium chloride and thiomersal. The nasal mucociliary clearance and in vitro ciliary beat frequency of nasal ciliated epithelium taken from healthy subjects were not adversely affected after topical application; neither did treatment with betamethasone, with or without neomycin, for four weeks affect these indices in two groups of patients with rhinitis. Thus, despite a ciliotoxic effect when ciliated epithelium is exposed to these preparations in vitro, they do not affect nasal clearance or ciliary beat frequency (as measured in vitro) adversely when administered topically to the nose.