The induction of HLA class II antigens on human thyroid epithelial cells has been shown to be an intimate part of the pathology of autoimmune thyroid disease and may also be relevant to the natural history of thyroid neoplasia since thyroid cancer cells may show spontaneous HLA class II antigen expression. We have, therefore, analyzed the regulation of HLA-DR-α-specific mRNA transcripts, as a model for HLA class II antigen induction, in three established human thyroid cancer cell lines (papillary thyroid cell line NPA, and follicular thyroid cell lines RO-82-W1 and MRO87-1). Each of the lines expressed 1.4 kb HLA-DR-α-specific mRNA transcripts, either constitutively or after cytokine induction, but showed markedly different regulatory characteristics. For example, the induction of HLA-DR α mRNA in response to recombinant human interferon-γ (IFN-γ) was inversely proportional to the degree of constitutive DR α mRNA expression; when there was constitutive absence there was a greater degree of induction. Similarly, the halflives of the HLA-DR α mRNA transcripts varied from 80 to 420 min with the longest degradation time occurring in those cells lacking constitutive expression of HLA-DR α mRNA. These data indicated that the degradation rate was not a determinant of their constitutive expression. We also sequenced the 5′ promoter region of the HLA-DR α gene (nucleotides -88 to -277 with respect to the translation start site) in each of the three cell lines. Sequence analyses revealed identity to the previously published normal human genomic sequence. Taken together, therefore, these data indicate that transactivating factors, rather than changes in mRNA degradation or promoter abnormalities, are the likely causes of variation in constitutive and cytokineinduced HLA-DR α gene expression in human thyroid cancer cells.