Polychlorinated biphenyl (PCB) exposure has been linked to a variety of toxic effects in animal experiments and in certain human case reports and epidemiologic studies. A total of 209 individual PCB congeners are possible, based on chlorination level and ring substitution pattern. Commercial PCBs are a complex mixture of congeners, and over 75 of these have been reported to be present in human tissues at widely varying levels. Because the biologic activity of individual PCBs is a function of extent and pattern of chlorine substitution, "congener-specific" PCB analysis of human tissue has gained increasing importance in assessing possible links between PCB exposure and toxic effects. A high-sensitivity analytical method using dual-column gas chromatography (GC) with electron capture detection (ECD) for determining 101 PCB congeners (83 individual, 18 as pairs/triplets) plus 1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene (p,p'-DDE), hexachlorobenzene (HCB), and mirex, in human serum is described. Separation is performed concurrently on parallel-configured DB-5 and Apiezon-L capillary columns. The current method is a modification of previously reported dual-column GC-ECD systems with improvements to the extraction and analytical protocols and the implementation of a comprehensive QA/QC program. The method employs two surrogate standards (PCBs IUPAC 125 and 192) and internal standard (IUPAC 104)-based quantitation, in addition to per-batch check standards and method blanks. Although optimized for serum, the method is applicable to all human, experimental animal, and environmental biota samples. The accuracy, precision, and reliability of the method were assessed using a variety of QA/QC endpoints. Finally, the use of the method in determining level and prevalence of PCB congeners in a cohort of adult Native-American individuals with historical environmental PCB exposure is reported.