Comparative investigations aiming to determine the most favourable dose for in vitro ultraviolet irradiation of cow milk which can ensure highest vitamin D content in it and best effect on vitamin D level in calf blood serum were carried out. It was established that under the applied technique of ultraviolet irradiation a 4 min treatment is most suitable and vitamin D content in milk rises several times (up to 8 times). Feeding of calves up to the age of 60 days with irradiated milk ensures a 4- to 5-fold increase of vitamin D in the blood serum as compared to its initial level before the experiment started and 2--3 fold as compared to that in control group calves.