Benzisothiazolin-3-one is a preservative widely used in industry. The manufacturer states that it is a known skin irritant and sensitiser, but these facts are not well-known. A case is described of contact allergy in a lithoprinter caused by it when used as a preservative in gum arabic. The literature is reviewed and a suitable patch-test concentration is suggested. It should be included in the patch-test battery for lithoprinters.