Neonatal hamster kidney cells (BHK21/C13), challenged in monolayer culture for three days with crude rat liver histone, have been shown to exhibit increased malignant characteristics when injected subcutaneously into hamsters. In contrast to the controls, the challenged cells produced tumours which invaded either the epidermis or the body wall of their hosts and frequently caused extensive visceral metastases. In vitro studies of the cell cultures, during and after histone treatment, suggested that cellular "transformation" rather than selection was effected by the crude histone preparation. Cells from the primary tumours of both control and test groups appeared morphologically identical but after sub-culture in vitro they retained their respective growth characteristics on reinoculation.