The frequency of Streptococcus mutans in plaque material from margins of conventional amalgam and non-gamma-2 amalgam fillings was examined in six subjects. One and 2 months after restorative treatment the mean values of the percentage S. mutans of the total number of microorganisms were somewhat lower in samples from non-gamma-2 amalgams than in those from conventional amalgams. The difference in values, however, was not statistically significant.