A defect of haemoglobin synthesis is the classically recognized mechanism affecting the erythron functionalism in chronic iron deficiency. The poor erythroblastic bone-marrow response, plus a number of dyserythropoietic nuclear features, have led to think of an impairment of the cell cycle of erythroblasts in iron-lack anaemia. The aim of the present work was to study such hypothesis, not proven so far. Ten subjects with normal haemopoiesis and 39 patients with iron-lack anaemia of different aetiologies (namely, 19 with digestive tract bleeding, 16 with gynaecological bleeding, and 4 with haemorrhages on other locations) were included in a previously reported protocol. The scheme of such protocol consisted of: 1) bone-marrow erythroblast quantification; 2) analysis of their maturation gradient; 3) erythroblast mitotic index; 4) measure of the mitotic time in bone-marrow culture; 5) tritiated-thymidine incorporation to short-term bone-marrow culture and quantification of the erythroblastic labelling index. To these were added the degree of nuclear dyserythropoiesis according to Hill and Lewis, and the reticulocyte production index. The following mean values were found in the control group: erythroblasts, 25.5 (+/- 3.63) %; E1-E4, 47.66 (+/- 3.09) %; IDN, 0.67 (+/- 0.27); MI, 2.82 (+/- 0.66) %; MT, 1.05 (+/- 0.15 hr); LI, 34.88 (+/- 5.82) %. The mean values found in iron-lack anaemias were as follows: erythroblasts 39.42 (+/- 9.1) %; E1-E4, 42.25 (+/- 4.11) %; IDN, 7.77 (+/- 4.69); MT, 1.81 (+/- 0.95) hr; LI, 13.08 (+/- 6.51) %. The statistical analysis (Student's t) showed highly significant differences (p less than 0.001) in the increased IDN and decreased MI and LI in iron deficiency patients.(ABSTRACT TRUNCATED AT 250 WORDS)