The possibility of storage of human bone marrow CD34+ cells at 4 degrees C was investigated for bone marrow transplantational purposes. The cells were placed in Iscove medium supplemented with 20% serum (15% bovine calf serum 5% human AB serum) for three weeks at 4 degrees C. During the storage time, clonogenecity of granulocyto-monocytic (CFU-GM) erythropoietic (BFU-E) and megakaryocytic (CFU-Meg) progenitors was investigated. It was found that it is possible to keep human CD34+ cells at 4 degrees C, at least for a few days before transplantation. At day four of the storage, the number of CFU-GM and BFU-E cells still exceeded 50% of the cells present at day 0. However, we have found that CFU-Meg in comparison to other progenitors are much more sensitive to metabolic storage stress. This enhanced sensitivity of megakaryocytic progenitors could explain, at least partially, the well known phenomenon of retardation of the thrombopoietic recovery in patients undergoing bone marrow transplantations.