Human respiratory mucosa and human granulocytes were exposed to topical nasal steroids in vitro. The preparations containing benzalkonium chloride and benzalkonium chloride alone destroyed the mucosa within 10 days. The same preparations also inhibited human neutrophil actin polymerization, degranulation and oxidative burst in vitro in a time and concentration dependent manner. Preparations without benzalkonium chloride, as well as the steroid compounds themselves, did not have these effects. It is concluded that benzalkonium chloride has toxic effects on human respiratory mucosa and human neutrophils in vitro.