This paper describes analytical methods to estimate environmental and occupational exposure levels of chromium in blood and serum by Zeeman atomic absorption spectrometry. Also reported is an internal quality control procedure involving a combination of an outline quality control and subsequent statistical evaluation of the quality control results to evaluate the performance of the analytical methods. The solubilization of the blood by the proteinase Subtilisin A® resulted in a recovery of chromium of 106 ± 4.5%; the uncertainty was ∼ 10% at a chromium level of 1 μg l^-1. the detection limit (LOD) for chromium in serum was 0.17 μg l^-1 and 0.20 μg l^-1 for chromium in blood. The LODs were sufficiently low for the determination of chromium in a large fraction of reference populations not occupationally exposed. The present study indicates that reference values for chromium in blood and serum are low but within the range in recent investigations, i.e. 0.04-0.35 μg l^-1 in serum and 0.12-0.34 μg l^-1 in human whole blood. The fraction of reference values below LOD was between 0.45 and 0.57 for chromium in serum and blood. Consequently, the reference populations were described by distribution free one-sided tolerance intervals and the precision of the estimation of the tolerance intervals was expressed as coverage intervals. The 95% one-sided tolerance limit calculated for chromium in serum was 0.60 μg l^-1 with the coverage interval (95 ± 4.8) percent at a probability of 0.95. Thus, the probability was 0.975 that the tolerance interval covers at least 90.2% of the distribution. In addition, the probability was 0.025 that the tolerance interval covers more than 99.8% of the population. It was only possible to calculate the 90% tolerance interval for chromium in blood with the coverage interval at 0.90 probability. The one-sided tolerance interval for chromium in blood was 0.37 μg l^-1 with the coverage interval (90 ± 9.9) percent at a probability of 0.90.