Cultures of human astrocytoma have been derived by collagenase digestion and are presumed, from their aneuploid karyotypes, to be predominantly neoplastic. Early passage cultures in proliferative phase have been cloned in the presence of dexamethasone and betamethasone, both commonly used in management of patients with brain tumours. These steroids raise both the cloning efficiency and the proliferative capacity of cells within each clone. Inhibition was detected only in very high steroid concentrations (25-50 microng/ml). Since these concentrations are unlikely to be attained in vivo it is concluded that anticipated physiological levels of these steroids enhance cell survival at low densities in culture. The significance of this in vivo is discussed.