Insulin-dependent cytoplasmic regulator (IDR), the activity of which correl
ates with the insulin level in blood, was isolated from rat liver and fract
ionated on a Sephadex G-25 column. Radioautography established that IDR sti
mulates by 30% proliferation of murine melanoma KML cells in culture. This
is indicated by H-3-thymidine incorporation into the cell nucleus. It is pr
oposed that insulin stimulates proliferation of the melanoma KML cell cultu
res through an intercellular intermediate, IDR.