Animal keratinocytes proliferate indefinitely in the presence of Y-27632. Growth curves of foetal bovine keratinocytes (A) or new-born mouse keratinocytes (B and C) grown in the absence or presence of Y-27632. Cells in A and C were grown in F-medium in the presence of J2 3T3 feeders. Cells in B were grown in low calcium DMEM in the presence of J2 3T3 feeders. The arrow represents continued proliferation and the X represents senescence. In panel C, the *symbol represents a stage when most cells were senescent; however, a few very rare cells spontaneously transformed and began to proliferate. Panels D, E and F show phase contrast images of bovine (D), mouse (E), and human keratinocytes cultured for the passes shown in the presence of Y-27632. DMEM, Dulbecco’s modified Eagle’s medium.