Prior to the onset of DNA synthesis, serum induces an array of "early events" (8-16) that do not require de novo protein synthesis. This array includes stimulation of the uptake of nucleosides (8-10), potassium (11), phosphate (8-10), and nutrients such as glucose (12, 13) as well as changes in cellular content of cyclic nucleotides (10,14). These early events are potentially important both as links, in the biochemical chain of events, leading to proliferation, and as markers for investigating the interactions of proliferative stimuli with their cellular receptors (15,16 In the final step not previously described, active fractions were pooled, neutralized, dialyzed against 5 mM sodium phosphate at pH 7.5, and applied to a 1 X 25 cm carboxymethylcellulose column (Whatman CM-52) previously equilibrated with the same buffer. After washing with 25 ml of column buffer, activity was eluted with a 140 ml linear gradient of 060 mM NaCl. In most preparations two peaks of activity were seen' the second and larger peak (eluting at 200-300 mM