1. Bone marrow of guinea pigs grows better in autogenic than in homogenic serum.
2. Bone marrow of guinea pigs grows more extensively and rapidly in unheated than in heated autogenic and homogenic serum (56° C. for half an hour).
3. In different heterogenic sera, there are differences in the growth of the tissue. In none of the sera examined, was the growth so extensive as in homogenic and autogenic serum.
4. Heated heterogenic serum is a better culture medium and shows more extensive tissue growth than normal unheated heterogenic serum.