The atomic absorption method described will determine total mercury and, selectively, inorganic mercury. Organic mercury is measured as the difference between total and inorganic mercury. Lower levels of mercury in samples of human blood (4.1 ng Hg/ml) can be determined with the following precision: total mercury 0.6%, inorganic mercury 8.7%, and organic mercury 5.6%. The mean deviation of 2 sets of analyses, one carried out with the method described in this paper and the other with neutron activation analysis, was 3.6%. Organic (presumably methyl) mercury accounted for 66% of the total mercury in blood in a population with minimum exposure. The red cell-to-plasma ratio of organic (methyl) mercury averaged 13.9 in a population with moderate exposure to mercury in food.