Thyroglobulin was isolated from the cattle thyroid gland by chromatography on Sephadex G-200 and Sepharose 4B. It was found homogeneous according to disc electrophoresis (pH 8.6) and analytical ultracentrifugation. However, gel filtration of thyroglobulin incubated at 37 degrees through Sepharose 6B revealed that it contained proteinases of the serine type. The conditions for the proteinases' inhibition were found. The possibility of obtaining thyroglobulin free of the proteinases by means of affinity chromatography is demonstrated.