In this paper the attempt has been pursued to quantify by a well planned experimental technique the ability of the neoplastic cells to spread around in vivo through various organs of laboratory animals. By making use of the histoautoradiographic techniques the incorporation of 3H thymidine either by the normal and by the neoplastic cells has been investigated at many different anatomical sites in Ehrlich ascites tumor-bearing mice. The method has been shown to be well suited also for the quantitative characterization of the spreading dynamics of the cancer cell population.