Summary The factor in normal and leukaemic mouse serum responsible for the stimulation of bone marrow colony growth in vitro has been characterised. It is a heat labile, non‐dialysable molecule which migrates electrophoretically in the α‐globulin‐post‐albumin region, is resistant to treatment with ether, RNA‐ase and DNA‐ase, has a buoyant density of approximately 1·34 and a sedimentation coefficient of between 4·5 and 7·0. It is likely that this factor is a protein or glycoprntein and that it may represent a Specific leucopoietic substance. The study also revealed the presence in various mouse sera of components which inhibited or potentiated the effect of the Factor in stimulating colony formation from bone marrow cells.
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Stanley et al. (1968) studied this question.