Liver tissue produces a potent lactogen that partially mimics the actions of prolactin.

An in vitro bioassay (based on the detection of casein release from isolated mammary cells by reverse hemolytic plaque assay) was used to detect a lactogenic factor secreted by liver tissue from suckled rats. This material stimulates casein release in the absence of added PRL and is actually more potent than PRL in this regard. The substance(s) possesses the following additional characteristics: it is released from liver tissue of lactating but not virgin female or male rats; when tested together with PRL, its effects are additive rather than synergistic; and unlike PRL, it does not increase the proportion of mammary cells committed to casein release. These findings are consistent with the possibility that a Liver Lactogenic Factor may mediate, at least in part, the lactogenic component of PRL's action. This functional relationship may be similar to that which exists between GH and the somatomedins.