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Embryonic rat fibroblasts which have been stopped in G1 by incubation in serum-free medium can be triggered to proceed through their cell cycle by the serum proteins S1 and S2. To determine the specificity of these serum factors, a wide variety of compounds have been tested: Proteolytic and lipolytic enzymes, hormones, serum proteins, prostaglandins etc. Slight effects could be de­tected with α2-Macroglobulin-Schultze and Prostaglandin F2α in combination with insulin. Stimulation of the cells comparable to that obtained with S1 can be achieved by treating the cells for 10 min with low concentrations of trypsin followed by incubation in serum-free medium containing insulin. The effect of trypsin can be completely abolished by soybean trypsin inhibitor.