On tumor NKLy/LL the author have studied the efficiency of cloning, the fraction of proliferating and resting cells and the rate of DNA synthesis in peripheral and central tumor sites. It was shown that the more far from the periphery the less fraction of labelled cells and the rate of DNA synthesis but the greater are differences in the content of proliferating cells in some sites. The efficiency of cloning in peripheral and central tumor sites does not differ, the fraction of clonogenic cells being larger than that of proliferating ones. Following persistent administration of 3H-thymidine in animals, it was found that the number of colonies produced by proliferating cells is considerably less than the general efficiency of cloning. It is concluded that resting cells can divide infinitely and are clonogenic ones.