Several adamantyl derivatives of thienyl phencyclidine (tenocyclidine; TCP) were newly sythesized and characterized: adamantyl derivatives containing piperidine (TAPIP), pyrrolidine (TAPYR), and morpholine (TAMORPH) groups. Their biological activity was evaluated by in vitro testing of their effect on the proliferative and reproductive ability (cytotoxicity) of a human tumor cell strain and nonmalignant mouse fibroblasts in culture. We also tested them for their radioprotective effect after ionizing irradiation, and as anticancer agents on the same human tumor cell strain. Compared with TCP, adamantyl derivatives are less toxic and have outstanding radioprotective properties. These derivatives (especially TAMORPH) increase apoptotic death of human malignant cells. The radiation-modifying effect studied on C3Hf mice in vivo showed that the adamantyl derivatives of TCP have a more enhanced radioprotective effect and that they are less toxic than TCP itself. The present data are discussed and compared with those previously reported for structurally related phencyclidine derivatives.