Twelve men developed contact sensitization to a neat cutting oil to which they were occupationally exposed in the same factory. Serial patch testing ultimately showed strong positive reactions to the chlorinated paraffin fraction of the oil in all 12 patients, whereas negative results were obtained in all 25 controls. Patch testing to pure chlorinated paraffin, on the other hand, gave negative results. Unexplained positive patch test reactions to epoxy resin from the standard series were elicited in half of the cases, suggesting that an epoxy stabilizer within the chlorinated paraffin fraction was the most likely sensitizer responsible for the outbreak of dermatitis. Because of insufficient manufacturer cooperation, we were not able to specifically identify the sensitizer. A number of deficiencies in the work practice, which accounted for the relatively large size of the sensitized group, were identified.