It was confirmed that the leaves of Acer platanoides contain an antifungal inhibitory substance. Low concentrations of sterile cold water extracts inhibited the germination of the spores of Cladosporium herbarum (three isolates), Cladosporium sphaerospermum and Cylindrocarbon radiclcola. In the concentration range 0·06–0·125 per cent (w/v) of leaf material the inhibitory response was demonstrated to increase linearly as the concentration of leaf material increased logarithmically. Inhibitory activity was found in leaf samples collected during a period from July to October but activity had disappeared from leaves collected in the following January. The inhibitory activity was located in two components of the water extract by bioassay tests following ether extraction and separation by chromatography. One of the active components has been identified as gallic acid by gas chromatography. Gallic acid has also been detected in dew collected from leaf surfaces where it is suggested that it may play an important part in the colonization of the leaves by fungi.