Thyroglobulin (TG), the major exportable protein of thyroid follicle cells, is conveyed to lysosomes on a complex secretion, storage and recapture pathway by as yet unknown transport mechanisms. This report establishes that the dimeric porcine TG-molecule carries an average of six phosphate residues. Endoglycosidase digestion showed that two phosphate residues are bound to the high-mannose carbohydrate side chains (CHO), while two others are linked to the complex CHO. These four residues are also sensitive to alkaline phosphatase treatment, indicating their terminal linkage. Immunoprecipitation analyses showed that TG obtained from microsomal fractions is already phosphorylated. Most important, an enzymatic assay applied to hydrolysates of TG established that the two phosphate residues at the high mannose CHO are present as mannose-6-phosphate (M-6-P). Alkaline phosphatase treatment of biosynthetically radiophosphorylated CHO followed by hydrolysis and t.l.c. indicated that M-6-P is present at least in part in phosphomonoester linkage. Furthermore, porcine TG binds specifically to the M-6-P receptor of Chinese hamster ovary cells. It is concluded that the M-6-P residues of TG are exposed and able to operate as a ligand for the M-6-P receptor. It is unknown why the lysosomal recognition-marker M-6-P does not convey TG directly on an intracellular route to lysosomes. We propose that for the secretion of newly synthesized TG into the follicle lumen an additional export signal dominating over the M-6-P recognition-marker is required.