By means of analytical procedures developed for separation of milk cellular components and for direct radioimmunoassay of thyroid hormones in milk the quantitation of thyroxine (T₄) monodeiodinating activity in the whole milk and in the milk cellular components has been achieved. Existance of both T₄ to T₃ and T₄ to rT₃ conversions activities have been demonstrated. The cows milk from animals displaying mastitis showed lower triiodothyronine (T₃) levels than controls. The T₃ content in milk was somewhat related to the bacterial agent responsible for mastitis. The presence of enzyme system, converting T₄ to T₃ and yielding biologically potent T₃, in mammary gland is considered as important for galactopoiesis.