Presence of a nonlactogenic factor in human serum which synergistically enhances prolactin-stimulated growth of Nb2 rat lymphoma cells in vitro.

A nonlactogenic factor(s) which synergistically enhanced the effect of ovine PRL in stimulating the growth of Nb2 rat lymphoma cells in suspension culture was found in human serum. The identity of the factor is unknown, but it is heat stable, with a mol wt apparently in excess of 8000 daltons. It possesses no growth-stimulating activity in the absence of PRL. Insulin acts in a similar manner to this factor in the Nb2 bioassay, suggesting that the factor may be somatomedin-like, possibly related to the previously described synlactin.