EN
The author studied the immunizing effect of artificial infection of animals both with fully invasive and X-ray irradiated larvae (40,000 r) of Dictyocaulus viviparus. The results obtained suggest that: 1. single dosis of 200-500 larvae per animal (fully invasive and irradiated ones) does not protect calves from natural invasion on the pasture, but it does attenuate the course of the disease: 2. the larvae administered twice in the above given amounts, at 5-week intervals. were found to immunize the animals against natural invasion so that after pasture infection they do not show any symptoms of the disease; 3. in control animals remaining under the same experimental conditions the course of dictiocaulosis was very severe.