In order to assess the quality and comparability of results from determinations of 1,2,5,6,9,10-hexabromocyclododecane (HBCD) in biological samples, two interlaboratory comparison studies have been organized. Up to 13 laboratories determined either the total HBCD concentration, or concentrations of alpha-, beta- and gamma-HBCD, or both in cod liver oil, herring filet, salmon filet, butter and chicken meat. The laboratories were able to determine total HBCD concentrations in the marine samples with satisfying quality (RSD <35%). However, the analysis of samples with low HBCD contamination (<about 2ngg(-1) lipid weight) should be improved. No statistically significant differences were found between total HBCD concentrations obtained by LC-MS and GC-MS. To our knowledge, this is the first report on interlaboratory comparison studies concerning the determination of HBCD in biological samples comprising results from a sufficient number of participants to perform a statistical evaluation.