The mechanism of the impaired response to thyrotropin (TSH) in thyroid tumor cells was investigated by searching for structural changes in the TSH receptor (TSH-R) in neoplastic thyroid tissues in humans. Total RNA was prepared from 34 thyroid tissue samples (four normal, six adenoma, six follicular cancer, and 18 papillary cancer) and reverse- transcribed into single-stranded cDNA, which was then used as a template for the polymerase chain reaction and subjected to single-strand conformation polymorphism (SSCP) analysis. Two fragments, FRAG (468-692) (nucleotides 468 to 692, corresponding to the mid-portion of the receptor extracellular domain) and FRAG (2044-2295) (nucleotides 2044 to 2295, corresponding to the COOH-terminal, cytoplasmic domain of the TSH-R cDNA) showed differences in electrophoretic mobility among the various thyroid tissue samples. Direct sequencing revealed Phe197 (TTC) --> Ile(ATC), and Asp219 (GAT) --> Glu(GAG) substitutions in FRAG (468-692) from two papillary cancers. Three types of substitution were identified in FRAG(2044-2295): Asn715 (AAC) --> Asp(GAC) from one papillary cancer, Lys723 (AAG) --> Met(ATG) from one papillary cancer, and Asp 727 (GAC) --> Glu(GAG) from one normal tissue sample, one follicular cancer, and four papillary cancers. These results suggest that there exist structural changes in TSH-R in some cases of thyroid neoplastic tissue.