A gas chromatographic-electron capture detection method for determining the concentration of polychlorinated biphenyls (PCBs) as Aroclor 1254 (AR 1254) in serum was evaluated through a 2-phase collaborative study. In Phase I, each collaborator's lot of Woelm silica gel (70-150 mesh) was evaluated for elution and recovery of AR 1254, which had been added in vitro at 25 ng/mL to a serum extract. In Phase II, each collaborator analyzed a series of bovine serum samples that contained the following: (1) in vitro-spiked AR 1254; (2) in vivo AR 1254 and 8 in vitro-spiked chlorinated hydrocarbons; (3) in vivo AR 1254 only; (4) 8 in vitro-spiked chlorinated hydrocarbons only; and (5) neither AR 1254 nor chlorinated hydrocarbons above the detection limit of the method. In Phase I, the average recovery of AR 1254 from silica gel for the 6 collaborators was 87.9 +/- 15.44% (mean +/- 1 SD; N = 18; range = 52.3-105.8%). In Phase II, the analysis of in vitro spikes of AR 1254 in serum at 8.58, 16.8, 41.8, and 84.3 ppb gave mean (means) interlaboratory recoveries of 89.0, 83.3, 79.4, and 76.9%, respectively, with within-laboratory (repeatability) relative standard deviations (RSDr) of 18.8, 20.5, 10.2, and 14.1%, respectively, and among-laboratory (reproducibility) relative standard deviations (RSDR) of 21.5, 21.1, 14.6, and 20.8%, respectively. The determination of in vivo AR 1254 in samples containing approximately 10, 25, 50, and 100 ng/mL of AR 1254 resulted in interlaboratory means of 10, 22, 39, and 79 ng/mL, respectively, with RSDr = 6.7, 9.7, 6.4, and 5.8%, respectively, and RSDR = 20.6, 16.0, 10.9, and 10.3%, respectively. The precision of the method for incurred AR 1254 showed a maximum RSDr of less than 10% and a maximum RSDR of less than 21% for a concentration range of 10-100 ng/mL. The accuracy of the method as demonstrated by the mean recovery of in vitro-spiked AR 1254 over a concentration range of 8.58-843 ng/mL was 82.2%. The method has been approved interim official first action.