It is generally accepted that cancer has strong effects on bone marrow cells (e.g. tumor-associated anaemia). The present investigation studies the proliferation kinetics of haemopoietic cells and the topographic differences in proliferation patterns in the bone marrow of tumor-bearing animals and controls. It was demonstrated that the differences in the cell cycle times, whether between peripheral and central parts of the bone marrow or between the experimental group and the control group, were slight or nonexistent. However, the growth fraction showed an increased proliferation rate in tumor-bearing as compared with normal animals (57% and 37%, respectively).