Thyroglobulin messenger RNA (mRNA) was located and quantified in tissue sections of differentiated human thyroid cancers by in situ hybridization using cloned complementary DNA probes. The cells of the well-differentiated follicular and papillary forms contained similar levels of thyroglobulin mRNA, corresponding to about 2000 copies per cell. In contrast, cells of moderately differentiated thyroid cancers contained about two to three times less thyroglobulin mRNA. It was also found that thyroglobulin mRNA was present almost exclusively in polyribosomes under the form of heavy polyribosomes actively synthesizing thyroglobulin. It is suggested that in situ hybridization method allows localization of specific mRNA in differentiated thyroid cancers and correlation with the level of differentiation of the cells.