In an electron microscope study of guinea-pig bone marrow, particular attention has been paid to the relation of lymphocytes to the sinusoidal endothelium. For the most part the endothelium consists of a single layer of cells unsupported by a basement membrane. The endothelium shows numerous fenestrations. At the junction of two endothelial cells, the cytoplasmic surfaces usually overlap and adhesion belts may be seen. Lymphocytes have been repeatedly observed not only lying in close relation to both aspects of the endothelium but also apparently in the act of passing through. Despite the thinness of the endothelium these lymphocytes may show a marked constriction in the middle. It is not possible from a study of the electron micrographs to tell in which direction the lymphocytes are moving. But whatever the direction of movement, the appearances seem clearly to indicate that lymphocytes do in fact traverse the sinusoidal endothelium and that there is at least some interchange of lymphocytes between marrow and blood.