The effects of heparin on the in vitro growth of rat cervical epithelial cells were examined. Heparin was found to inhibit in a dose dependent fashion the log-phase growth of rat cervical epithelial cells (RCEC) grown in the absence of medium supplements. An inhibition of growth is observed at concentrations as low as 500 ng/ml and 50% inhibition of growth occurs at a concentration of 5 micrograms/ml. The growth inhibitory activity of heparin is independent of anticoagulant activity since three separate non-anticoagulant preparations of heparin all inhibit growth. Other glycosaminoglycans including chondroitin 4-sulfate, chondroitin 6-sulfate, dermatan sulfate, hyaluronic acid, and keratin sulfate do not inhibit the growth of rat cervical epithelial cells. The ability of heparin to inhibit the log-phase growth of rat cervical epithelial cells is dependent on the composition of the medium in which the cells are grown. The addition of greater than or equal to 7.5 ng/ml epidermal growth factor to epithelial cultures blocks the growth inhibitory activity of heparin. These results suggest that components of the extracellular matrix modulate the growth responses of epithelial cells and may be important in regulating cellular proliferation in normal and pathological states.