Experiments in a standardized rabbit model indicate that the yellow-golden stain formed on tooth surfaces during the use of SnF2 may consist of stannic sulfide. The nature of the stain, the presence of sulfur in the samples and the effect of an oxidizing agent on the stain support this view. It is suggested that the low pH of SnF2 caused denaturation of pellicle protein with exposure of sulfhydryl groups, which later form stannic sulfides through reactions with the stannous ions present in the preparations.