Preparations of 2‐n‐octyl‐4–isothiazolin‐3‐one (Skane M‐8® at 1000 p.p.m. a.i. and 500 p.p.m. a.i. in pet. induced allergy in a cumulative insult human patch test. Challenge patch testing using 250 p.p.m. a.i. Skane M‐8 elicited allergy in 6 out of 6 sensitized individuals. Sensitized individuals did not cross react with Kathon® biocide, (Kathon CG). a mixture of 5–chloro‐2–methyl‐4–isiothiazolin‐J‐one and 1 methyl‐4–isothiazolin‐3–one. All pretrolatum preparations to which Tween 85® was added at 2.5% produced mild irritation in cumulative insult patch tests, but no irritation was observed with Tween 85 at 0.625%. We conclude that Skane M‐8, 250 p.p.m. a.i. with Tween 85 0.625% in petrolatum is appropria