lic acid spacers at para-position were derived from coplanar 3,3,4,4-tetrachlorobiphenyl and 3,3_,4,4_,5-pentachlorobiphenyl (IUPAC No. 77 (PCB 77) and 126 (PCB 126), respectively). It appeared from the results that the length of the spacer did not affect significantly the sensitivity of the assays established. The direct ELISAs using the antibodies in combination with various hapten–peroxidase tracers showed IC50-values for polychlorinated biphenyl (PCB) 77 and 126 of 1.3–19.2μg l−1. The ELISAs were tested for their cross-reactivity profiles using 20 selected congeners. The assays were highly specific for non-ortho-substituted (coplanar) congeners and did not recognize the more abundant but less toxic non-coplanar PCBcongeners or organochlorine compounds. The superior assaywas found for the antibody againstPCB126-(CH2)3-hapten mimic–bovine serum albumin (BSA) in combination with PCB 77-CH2-hapten mimic-peroxidase tracer. The IC50-values for the assay to PCB 77 and 126 were 2.0 and 5.2μg l−1, respectively; the highest cross-reactivity values (PCB 126=100%) achieved for non-coplanar structures and metabolites were only 1.3 and 1.0% for 2,3,3,4,4,5,5 heptachlorobiphenyl (PCB 189) and PCB 77 ether, respectively. In order to interpret measured data in terms of analytical or toxicological equivalents, a reliable relationship between assay responses and the coplanar congener content of Aroclor-contaminated samples should be verified and validated.