OBJECTIVE: To assess the proficiency of commercial laboratories in analyzing lead in clinical blood samples from subjects without overt lead exposure. DESIGN: We submitted masked duplicate blood lead specimens to 8 masked laboratories. Each laboratory received blood aliquots immediately following drawing (time 1) and 2 weeks later (time 2) from 7 human subjects and 3 bovine blood samples with known lead levels of 0.26, 0.57, and 0.79 micromol/L (5.4, 11.8, and 16.4 microg/dL). Of the 8 laboratories, 5 were commercial laboratories, 1 was a state laboratory, 1 was a research laboratory, and 1 was the Centers for Disease Control and Prevention reference laboratory. OUTCOME MEASURES: Correlation coefficients were calculated, and differences within and between laboratories were assessed by analysis of variance. RESULTS: Results were obtained for all specimens, with all the human subjects' overall mean lead levels being less than 0.48 micromol/L (<10 microg/dL). Each laboratory reported all human blood specimens appropriately, as having lead levels less than 0.48 micromol/L (<10 microg/dL) and within 0.14 micromol/L (3 microg/dL) of the overall mean for that subject. All internal reproducibilities were very high (range, 0.92-1.00) except for one (0.60), possibly lower because of 1 pair of specimens. Mean differences between blood samples analyzed at time 1 and time 2 ranged from -1.4 to 1.2, with only 2 laboratories having significant differences (P<.01). CONCLUSIONS: Overall, there was strong reproducibility within and among laboratories, with no overall time trend or interlaboratory or intralaboratory variance. The storage conditions did not seem to affect the aggregate results. The data suggest that through implementation of the Centers for Disease Control and Prevention/Wisconsin Blood Lead Proficiency Testing Program, the Centers for Disease Control and Prevention's Blood Lead Laboratory Reference System, and mandated federal and state proficiency programs, laboratories in this geographic region have improved their performance as compared with previous published studies and an unpublished study.