Abstract Twelve healthy volunteers (4 women and 8 men aged 23 to 45 years) were assessed for biological variation in copper, zinc, and selenium indices. Blood was drawn from these individuals once a week for 12 weeks and then once a month for a further 3 months. Over 12 weeks the mean plasma concentrations of copper, zinc, and selenium (measured by atomic absorption spectrometry), were 16 ± 2.9, 14.1 ± 1.5, and 1.28 ± 0.13 μmol/L, respectively. The intraindividual and interindividual coefficients of variation (estimated using analysis of variance (ANOVA) techniques) were 11 and 14% (zinc), 8 and 19% (copper), and 12 and 14% (selenium), respectively. The analytical goal for imprecision was achieved for all three micronutrients, i.e., it was less than one-half of the measured intraindividual variation. The indices of individuality for all three elements indicated that an individual's results as reference values are more useful than population-based data. The critical difference for significance between serial results is relatively smaller for copper (23%) and zinc (30%) than that for selenium (35%). The monthly mean concentrations of the three micronutrients over 6 months demonstrated no seasonal pattern.