Contact inhibition of proliferation. A. Proliferation of normal human skin fibroblasts (SF) in vitro. B. Proliferation of human liver connective tissue cells (LCTC). Cells were plated at 1 × 103 cells/cm2. Each day, during a period of 8 days, cells were trypsinized and counted in a hemocytometer (n = 3). Medium containing 10% serum was replaced every two days. C. [H3]-Thymidine incorporation in SF and LCTC at increasing cell density (n = 4).