Autologous lymphocyte populations from different phases of chronic granulocytic leukaemia (CGL), acute myeloid leukaemia (AML) and preleukaemic disorders were compared for cytotoxic activity. 51Cr-release tests showed that T lymphocytes from the quiescent phase of CGL and from the remission phase of AML exerted cytotoxic activity against autologous tumour cells. Such activity was also found in patients with potentially preleukaemic haematological disorders characterized by cytopenia, but not in polycythaemia vera patients. In the majority of cases cytotoxic activity of T lymphocytes could be blocked by native gp70 antigens of gibbon ape leukaemia virus (GaLV) and baboon endogenous virus (BaEV). Blocking effect of carbohydrate-free gp70 as well as p15(E) antigens could be observed less frequently. The role of cell-mediated immune response to oncovirus antigens in the course of myeloproliferative diseases is discussed.