In the department of prosthetic dentistry at Groningen, during the past four years, approximately 2,000 persons have been treated with Scutan, an epimine plastic for temporary crowns and bridges and Impregum, a polyether impression material. A total of ten patients complained of untoward reactions. Patch tests with the basic materials were performed on four of them. Positive allergic reactions were obtained to patch tests with the catalyst methyl p, toluene sulfonate 0.1 % in dibenzyltoluol present in Scutan, and the catalyst methyldichlorobenzene sulfonate 0.1 % in dibenzyltoluol present in Impregum. Four control subjects did not react to these concentrations.