Four hundred and sixty-six "blind" duplicate serum samples were analyzed for polybrominated biphenyls (PBB) by gas chromatography in two laboratories. As calculated by the regression model of Deming, the correlation coefficient for these 466 samples, whose values ranged from nondetectable amount to 1240 parts per billion was 0.9983. Prior to these analyses, the method was validated in both laboratories by using in vitro and in vivo PBB serum pools.