Hemopoietic activity in the spleen is caused by the proliferation of the RE cells of the red pulp, lining the sinusoids and the endothelial cells lining the venous channels and their vasa vasora. The RE cells of the marginal zone do not contribute. As the cells involved line blood spaces, and the immature cells are washed into the circulation hemopoiesis appears to be intravascular. Different agents produce different effect: copper sulphate an early proliferation of both the red pulp and venous cells, phenylhydrazine a red pulp proliferation and antiserum a high venous endothelial activity.