The action of 1., 2., 3., 4. and 5.000 r Röntgen radiations was experimented on the migration, survival and mitotic activity of in vitro hanging drop cultivated fibroblasts. Radial migration area was decreasing continuously, with increasing radiation doses, from the 4th day till the 12th day. After 12 days migration speed returned to normal and all cultures survived indefinitely. Mitosis number increased remarkably 5' after irradiation, reduced more and more after 15' and 30' with minimum value after 1h; then increased again. Mitotic activity became normal again after some transplantations.