Thyreostats are banned compounds in Europe since 1981 (directive 81/602/EC) because of their carcinogenic and teratogenic properties. The control of their illegal use in breeding animals is amongst the most difficult because of their low molecular weight, high polarity and the existence of tautomeric forms. The analytical procedure described in this paper involves, after a derivatisation step with 3-iodobenzylbromide, the detection and identification at low level (microg/L) by LC-ESI(-)-MS/MS of eight thyreostats in various biological matrices (urine, muscle, liver, thyroid, animal feed, faeces and hair). The urine method has been validated according to the EU criteria (2002/657/EC decision): CCalpha and CCbeta were found in the range (0.1-5.2 microg/L) and (2.6-23.2 microg/L), respectively. The performances fulfil the requirements of the EU regarding the provisional minimum required performance limit (MRPL) fixed at 100 microg/L in urine.