Haematopoietic tissue proliferates by the mitotic activity of the youngest cells. The first data on the proliferation of blood cells recorded by use of the stathmokinetic technique was followed by more detailed information obtained with autoradiographic studies and by using special precursors of DNA such as thymidine. It has been observed that in the bone marrow in progressive myeloid and erythroid maturation, proliferative capacity decreases progressively until it stops altogether at the myelocytic and polychromatic erythroblastic stage. For chronic myeloid leukaemia the labelling index of each proliferating cell type is similar to the values of the corresponding normal cells. In 1958 we used tritiated thymidine to study many cases of acute leukaemia, that is those with a complete differentiation block and a very high blast content, and we found a clear fall in proliferative capacity of these blast cells. The same results have been obtained in other laboratories and now, quite the contrary to what was thought a few years ago, namely that leukaemic cells grew faster than normal cells, we know that the rate of growth of these cells is often much slower than the corresponding normal cells.