Natamycin, a fungicide marketed as Tymasil, is claimed to reduce house dust mite numbers and would therefore be expected to improve asthma in children with mite sensitivity. We have tested this assertion by a double-blind, placebo-controlled trial. There was no significant effect on levels of Der p I in mattress dust between active and placebo groups at the end of the spraying period. Histamine inhalation challenge PC20, clinic visit symptom scores and lung function tests reflecting either large or small airways obstruction were also unchanged. Therefore this product is not a therapeutic option for mite-allergic patients using the manufacturer's recommended dose and method of administration. Other factors influencing the Der p I levels were also investigated. Of these, only month of measurement and bedroom wall humidity showed any association.