28 patients reacting to a PVC adhesive tape used in routine patch testing gave positive reactions to diphenylthiourea (DPTU) and to phenylisothiocyanate (PITC) in low concentrations. DPTU is a heat stabilizer in the PVC and is partly decomposed to PITC. The two substances were also found in another type of PVC. Thioureas giving isothiocyanates are also used in certain types of rubber. Isothiocyanates are present in plants, mainly of iheCruciferaefamily, and are also formed from thiuram sulfides and thiocarbamates. The connection between isothiocyanates derived from different sources should be investigated.