The efficacy of 4,5-dichloro-2-n-octyl-3(2H) isothiazolone (C9211) was determined using fungi isolated from food and drink processing plants. The anti-fungal performance of urethane oil based paints containing C9211 was tested in (a) laboratory experiments using a humidity cabinet and (b) field trials at a UK maltings establishment and in Singapore. The surface of the hard glossy paints without biocide proved to be inhospitable for the germination and growth of mould fungi in exposures in the humidity cabinet and reasons for this are discussed. In filed trials, paints without biocide developed extensive mould growth after several months exposure whilst all paints containing C9211 remained mould-free throughout the 18 month trial