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We describe a sensitive method for the determination of vanadium in biological substances by atomic absorption spectroscopy. By using background correction and pyrolytically coated graphite tubes with flameless apparatus, this vanadium assay has a sensitivity of 65 pg and a detection limit of 30 pg, approximately sixfold more sensitive than previously reported assays involving this technique. The vanadium concentrations have been determined in several biological materials, including bovine liver and human serum. To investigate signal interferences, we have determined vanadium concentrations in matrix solutions containing several elements at physiologically relevant concentrations. Evaluated by the method of standard additions, the vanadium signals are linear with concentration over a range of 3 to 100 micrograms/L. Tissue vanadium concentrations are similar to those previously reported. Serum vanadium concentrations can be determined directly, and we found values ranging from 2.22 to 3.94 micrograms/L in human serum. The improved sensitivity permits the use of sample sizes substantially smaller than previously required.