Skin exposure to biocides containing high concentrations of methylchloroisothiazolinone/methylisothiazolinone (MCI/MI) may cause severe chemical burns and may also induce sensitization. We report two cases in which skin exposure to a newly launched biocide containing 2-methyl-4-isothiazolin-3-one (MI) and 1,2-benzisothiazolin-3-one led to sensitization to Ml, which in the second case was preceded by a chemical burn. A study was performed to investigate the pattern of reactivity to MCI and Ml in two patients who presumably had a primary sensitization to Ml and in one patient who had been sensitized to MCI/MI by being patch-tested. The patients were patch-tested with serial dilutions of MCI/MI, MCI, MI, and 2-n-octyl-4-isothiazolin-3-one. The first two patients reacted to both MCI/MI and the separate active ingredients, with a higher level of reactivity to Ml than to MCI. The third patient reacted to MCI/MI and MCI only. A biocide containing Ml caused sensitization and occupational contact dermatitis in the first two patients, through contact with wallpaper glue in one case and after a chemical burn in the other case.