A certified reference material. NIMD-01, was developed for the analysis of mercury speciation in human hair. We collected the hair of Vietnamese males from a barbershop in Hanoi in 2016 and prepared 1200 bottles containing 3 g of sieved and blended hair powder. The certified value was given on a dry-mass basis, with the moisture content obtained by drying at 85°C for 4 h. Certified values with the expanded uncertainties (coverage factor, k = 2) were as follows: methylmercury, 0.634 ±0.071 mg kg ^-1 as mercury; total mercury. 0.794 ± 0.050 mg kg ^-1; copper, 12.8 ± 1.4 mgkg ^-1; /inc. 234 ± 29 mg kg ^-1; selenium, 1.52 ± 0.29 mg kg ^-1. An indicative arsenic concentration of 0.17 ± 0.03 mg kg 1 was measured. Extended uncertainties were estimated by sample homogeneity, long-and short-term stabilities, and a characterization from measurements made by collaborating laboratories.