Erythropoietic cell proliferation of 6 cases of different clinical states of β--thalassaemia (thal) was studied by a combined method using Feulgen-cytophotometry and autoradiography after in vitro labelling with 3H-TdR. An abnormal distribution of the DNA content and 3H-TdR incorporation was found, which was more pronounced in the one patient with thal major than in the 3 patients with thal minor and was not provable in the subclinical form (minima), indicating a direct relationship to the ineffective erythropoiesis and the states of anaemia in this disease. This proliferation disturbance consisted of an accumulation of cells in G1, a decreased proportion of cells in S and a decreased S/G2 ratio, and was limited to the early polychromatic cell compartment. In the basophilic erythroblasts, a decreased proportion of cells in G1 was observed revealing a shortening of the G1 period in this cell compartment