Haematopoietic cells of the rat bone marrow were killed almost completely by using hydroxyurea (OHU), cytoslne arabinoslde (ara‐C),60Co irradiation and busulfan. The relation between the regeneration of haematopoietic cells of the bone marrow and the behavior of the bone marrow lymphocytes and related cells was investigated quantitatively.Bone marrow lymphocytes and related cells were cytologlcally classified Into lymphocytes, transitional cells and blastic cells on the smeared preparation.Comparing the results of the experiment using OHU 2.5 g/kg with those using whole body irradiation of60Co 700R, the absolute number of nucleated cells In the bone marrow was reduced more prominently in the former case than in the latter. Nevertheless, regeneration of haematopoietic cells began earlier In the former case than in the latter.In the experiment using OHU 2.5 g/kg, haematopoietic cells in the bone marrow almost completely disappeared promptly after the treatment. On the contrary, the bone marrow lymphocytes were rather well preserved, making up 59% of control on the 1st day and 54% on the 2nd day. Moreover, it was evident that the decrease in number of bone marrow lymphocytes and the increase in number of transitional and blastic cells occurred simultaneously around the 4th day after the last dosage, just prior to the haematopoietic regeneration.On the 8th day after whole body irradiation with60Co 700R, in parallel with the appearance of haematopoietic regeneration, the bone marrow lymphocytes, transitional cells and blastic cells were the predominant populations In the regenerating bone marrow.In the experiment using ara‐G 600 mg/kg, the number of transitional cells increased evidently prior to haematopoietic regeneration.The author concluded that regeneration of bone marrow haemato‐poiesis can take place earlier and more rapidly, when the bone marrow lymphocytes are provided In sufficient nu