Using non-quantitative reverse-transcription polymerase chain reaction (RT-PCR), we found that thyroid peroxidase (TPO) is expressed in all differentiated thyroid carcinomas examined, although the ratio of the shorter to longer transcript is decreased in tumors that had lost the iodide concentrating capacity. TPO expression is lost in several thyroid carcinoma cell lines (TPC-1, 8305C, 8505C) and altered in another (TC-80). Nucleotide sequencing of the PCR products revealed missense polymorphisms in the TPO gene. Four out of five samples tested are heterozygous for TPO alleles in exon 15, showing both C and T at nucleotide 2612 (GTG coding for Val, GCG for Ala). One tumor is homozygous for T at this position. In exon 8, three samples show T at nucleotide 1189 (TCG, Ser) and C at 1265 (ACC, Thr), while most published sequences report G at both positions (GCG coding for Ala at 1189 and AGC coding for Ser at 1265).