Previous results have indicated that addition of putrescine to cultures of human fibroblasts stimulates cell proliferation. It is shown in the present paper that this is mainly due to a shorteining of the cell cycle. In the presence of added putrescine the cell cycle was 4 hours shorter than in the control. This result was obtained both from analyzing the fraction of labeled mitoses (FLM) curves and from counting the mitoses in synchronous cultures. The FLM curves further revealed that the shortening of the cell cycle was caused mainly by reduction in the length of the time required for DNA synthesis. It was about 30% shorter in the presence of added putrescine than in the control. The fact that addition of putrescine results in shortening of the S-period, and that this is to a great extent responsible for the increased rate of cell proliferation, distinguishes putrescine from other known growth factors.