Point mutations occurring at codon 201 of the gene coding for the alpha subunit of the stimulatory G protein impair intrinsic GTPase activity and lead to a constitutive activation of adenylate cyclase. We have examined thyroid follicular and papillary carcinomas and follicular adenomas and found samples that bear this mutation at codon 201 of the Gs alpha gene. Both mutation-positive and mutation-negative tissue samples were investigated for the level of Gs alpha expression relative to a pool of normal thyroid tissue, using immunoblotting against two (mid-region-specific and C-end-specific) antipeptide antibodies. Using 8000 g and 100,000 g membrane fractions of homogenized tissues we have demonstrated that the Gs alpha proteins in normal ad neoplastic thyroid tissues are represented by three isoforms: 43 kDa, 45 kDa and 52 kDa. We have quantified and compared the amount of Gs alpha protein and find it is overexpressed in mutation-bearing tissue samples.