On the Nature of Castle’s Hemopoietic Factor

1. Normal gastric juice (intrinsic factor) and vitamin B₁₂ together form a thermolabile hemopoietic factor which ripens megaloblasts in vitro, both gastric juice and B₁₂ alone being inactive.

2. The hemopoietic factor in normal serum which ripens megaloblasts in vitro also appears to be thermolabile, heating to 56 C. for 2 hours destroying some of its activity.

3. The relationship of these factors is discussed and an extra-gastric as well as a gastric source of intrinsic factor is postulated.