Square standard cavities were prepared in human teeth and restored with amalgam obtained from alloys of different trade-marks. Twenty-four hours after condensation, excess amalgam was removed from three of the four margins, each with a different rotatory instrument (number 4 bur, multiblade finishing bur, and fine green stone). No rotatory instrument was used on the fourth margin (control), and this control was polished with pumice and zinc-oxide pastes and Sweeney's brushes. Vickers' tests for micro-hardness of the amalgam were performed on the four margins of the restorations. The restorations were then submitted to metallographic polishing, and a second micro-hardness measurement was made seven days later. The statistical analysis of the data showed that micro-hardness values were higher at the margins on which no rotatory instruments were used. The lowest micro-hardness values were found at the margins where finishing and/or round burs were used. Polishing with fine green stones gave intermediate values.