1A factor involved in the causation of mammary tumours in mice, transmitted from mother to offspring in milk (later identified as the mouse mammary tumour virus) (now historical).
2Any of various incompletely characterized substances present in milk that have some specific physiological effect.
Origin
1930s.
Definition of milk factor in US English:
milk factor
noun
Medicine
1A factor involved in the causation of mammary tumours in mice, transmitted from mother to offspring in milk (later identified as the mouse mammary tumour virus) (now historical).
2Any of various incompletely characterized substances present in milk that have some specific physiological effect.