The aim of this work was to determine the quantity of remaining unreacted methacrylate groups in polymerized dental sealants. Eight brands of sealants were investigated by means of a grating infrared spectrophotometer. The infrared absorbance measurements were made before polymerization and repeated after the sealants had been polymerized at 37 degrees C for 24 h. The quantities of remaining unreacted methacrylate groups were determined and the data expressed as percentages of the total amount of methacrylate groups in the unpolymerized materials. The results demonstrate that commercially available sealants exhibit different degrees of conversion 24 h after the start of polymerization. The quantities of remaining methacrylate groups after polymerization under conditions comparable with optimal clinical conditions range from approximately 15% to approximately 35% in the different brands.