Peculiar cytoplasmic connection between erythroblasts was observed in the smear preparation of human bone marrow; a pair of erythroblasts in the resting stage was connected by "cytoplasmic bridge" which is very thin and sometimes long compared with the telophase bridge appeared in the usual cytokinesis. Electron microscopy revealed that small amount of microtubules were running along in the cytoplasmic bridge but mid-body was not seen. Plasma membrane about the middle of the bridge bulged to form "bulging ring". Erythroblasts in various differentiating stages were connected by the cytoplasmic bridge and the stage of each pair was almost the same. The occurrence of the cytoplasmic bridge between erythroblasts was evaluated in the smear preparations of human bone marrow with hematological diseases or normal conditions. In the normal bone marrow, the mean value was 8.1 +/- 3.6% (n = 33). In patients suffered from autoimmune hemolytic anemia, it was 4.3 +/- 2.3% (n = 12), aplastic anemia, 3.2 +/- 2.1% (n = 6) and paroxysmal nocturnal hemoglobinuria, 5.0 +/- 1.4% (n = 3). While in the erythroleukemia the occurrence was very low showing 0.2 +/- 0.1% (n = 3) of total erythroblasts. In MDS, RAEB (2.5 +/- 2.4%) and RA (2.9 +/- 2.2+%) showed statistically significant lower occurrence. In the normal bone marrow the occurrence was higher, so the formation of cytoplasmic bridge could be essential for a normal proliferation of erythroblasts.