1. After an oral dose of (14)C-labelled 3,3',4,4'-tetrachlorobiphenyl (CB-77), the conventional germ-free and bile-duct cannulated male Sprague-Dawley rat excreted approximately 80% of the dose in faeces and/or bile within 3 days. 2. For the germ-free and conventional rat, 15% of the dose was excreted via the faeces as metabolites covalently bound to lipids. Bile-duct-cannulated rats excreted similar amounts of lipid-bound metabolites in the bile. The lipid-bound metabolites appear to be formed in the liver and excreted via the bile, and the microflora did not seem essential for the formation of lipid-bound metabolites. 3. The novel CB-77 metabolites had chemical and physical properties similar to those of lipids with regard to solubility and polarity, as determined by partition characteristics on various chromatographic systems. 4. In addition to identification of hydroxylated CB-77 metabolites, several fatty acid esters of hydroxy-chlorobiphenyls were indicated and one hydroxy-tetrachlorobiphenylol palmitoate was identified, but fatty acid esters were minor metabolites. 5. Approximately 70% of the lipid-bound metabolites were present in the fraction that contained phospholipids. The formation of lipid-bound CB-77 metabolites seems a spontaneous reaction rather than an enzymatically catalysed reaction, as indicated by the large number of different lipid-bound metabolites.