Electron and light microscopical analysis of rabbit bone marrow and erythroid spleen clones from lethally irradiated mice injected with singeneic bone marrow revealed marked similarities between the process of nuclear expulsion from the late erythroblast and mammalian cytokinesis. Polysome disaggregation noticed by other authors in tissue culture cells in metaphase, was found also to exist in erythroblasts in mitosis as well as in the late stages of nuclear expulsion. In both systems erythroblasts in mitosis and in the nuclear expulsion process, a reduction in the capacity to incorporate amino acids was demonstrated by autoradiography. Analysis of the erythroid cell populations of spleen clones in mice treated with actinomycin D indicates that inhibition of RNA synthesis does not inhibit nuclear expulsion. The appearance and location of centrioles in late erythroblasts prior and during nuclear expulsion indicates that, as in cytokinesis, they may play a role in the expulsion. Deformations appearing in late erythroblast nuclei and cytoplasm after treatment in vivo with Colcemid support this hypothesis.