Continuous lymphoblastoid cell cultures were established after transformation invitro with HVA of marmoset splenic or circulating lymphocytes. Transformation was achieved by co-cultivating lymphocytes with lethally X-irradiated, HVA-carrying cells (derived originally from tumour cells of a marmoset experimentally infected with HVA) or by infecting marmoset lymphocytes with cell-free virus. Tenty-eight continous cultures from 39 that underwent co-cultivation and two of four lymphocyte preparations infected with virus became transformed. Cultivation periods before transformation were in the range 17-32 days (co-cultivation) or 51-53 days (cell-free virus). HVA genome expression in transfromed cultures was demonstrated by:(1) recovery of small amounts of HVA from culture fluids; (2) ability of lymphoblasts to induce infectious centres after co-cultivation with permissive cells; and (3) observation of antigen-positive cells after staining with monospecific antiserum. Most cultures were composed of T lymphocytes: cells of 16 of 24 cultures formed rosettes with sheep erythrocytes and none of 30 possessed membrane Ig.