Three round robins, involving 22 laboratories from eight countries, were conducted at six month intervals. Each of the participating laboratories analysed four samples of NIES-13 in three subsequent round robins during July 2000–October 2001. The objective of the current study was to determine accuracy and precision among participants for the analysis of total mercury. Both accuracy and precision for total mercury determinations in powder hair samples were good. The median within-laboratory (within-run) CVs ranged from 3.1 to 3.9% for four CRM samples. Most laboratories showed a proportional bias relative to the consensus mean of up to 16%. Two laboratories reported results that, on average, were almost 30% higher than the consensus mean value. Significant among-laboratory imprecision was found in the present study. Improvements are needed to reduce the analytical imprecision in a few laboratories, and attention must be focused on calibration issues and methodology-related problems.