[en] A cell surface heparan sulfate proteoglycan (HSPG) released by treatment of a rat hepatocyte cell line with 2mM inositol-2-PO₄, was isolated from log and confluent cultures. At 1.0μg/ml the HSPG from confluent cells, but not that from log phase cells, inhibited cell division when added to log phase cultures. After a time equal to one cell cycle, the treated cells resumed logarithmic growth. Continuous administration of HSPG prolonged the delay in cell division. When growth resumed, the cells grew to confluence and exhibited contact inhibition. Cultures were synchronized by a thymidine/aphidicolin block at the G1/S boundary. When HSPG was added with release of the block, the cells progressed through S and the first mitosis. However, the second round of DNA synthesis and the mitosis were delayed for a period equal to one cell cycle. Studies of the uptake of (³⁵SO₄)-HSPG by synchronized cells showed internalization and accumulation of HS in nuclei after release of the block and then disappearance of nuclear (³⁵SO₄)-HS at the first mitosis. During the lengthened G1 phase following mitosis, (³⁵SO₄)-HS reappeared in the nuclei. A similar pattern of cell cycle dependent appearance and disappearance of nuclear HS was observed in studies of ³⁵SO₄^2- incorporation into HSPG by synchronized cells