A food contamination incident involving polychlorobiphenyls (PCBs) and PCDD/Fs occurred in 1999 in Belgium. On heavily affected farms, concentrations of PCBs in chicken or pork fat exceeded the Belgium tolerance limit of 200 ng/g lipid weight for the sum of seven marker PCBs. Analysis of contaminated samples showed that the patterns for PCB and PCDD/F congeners differed among feed, chicken fat and pork fat. Lower chlorinated PCBs and polychlorodibenzofurans (PCDFs) including those with high TEFs (PCBs 105, 118, 126 and 2,3,4,7,8-PeCDF) were shown to either bioaccumulate more in chicken fat or to be eliminated more readily in pork. This leads to the possibility that consumption of chicken would result in a higher TEQ human body burden than that from the same consumption of pork. In addition, PCDF congeners with non-2,3,7,8-substitution (e.g., 1,2,4,7,8-PeCDF) were present in chicken fat but absent in pork fat. Since the residue pattern in this commercial episode changes less in the avian species, these results reinforce the value of birds rather than mammals as markers of the source of contamination with persistent organochlorine pollutants.