A selective inhibitor of cell proliferation from normal serum.

A factor in normal serum that selectively and reversibly inhibits proliferation of cells in culture has been enriched 160-fold from calf serum by sequential ammonium sulfate precipitation, gel filtration, and lectin-affinity chromatography. DNA synthesis of normal (but not transformed) rat hepatocytes, human lymphoblast lines, and mitogen-stimulated murine spleen cells is inhibited by greater than 90%, and Vero, murine myeloma, MELC, and a human colon carcinoma cell line to a lesser extent. Growth of other cell lines tested was not affected. Responsive cells are arrested apparently in G1 by this inhibitor, the effect of which is maximal by 24 hr and is spontaneously reversible thereafter unless it is renewed. The active fraction is a protein that migrates with the alpha 2-globulins; it is not a lipoprotein, and it is of high apparent molecular weight.