Se-Ax is a continuous mature T-cell line that we have established from a patient with Sezary's syndrome. An important finding was that the Se-Ax cell line required human serum for initial growth. Here we show that transfer of the permanent cell line to a medium deficient of human serum induces production of C-type retroviral-like particles with a unique morphology. Ultrastructurally, these particles are 120 nm in diameter with hexagonal shape, and show a small, centrally located round core of 30 nm. They are observed only extra-cellularly; typical budding, however, is not found. Both by serological testing and molecular analysis we substantiate the morphological conclusion that these particles do not represent common human or animal retroviruses. The inducibility of the particles may be a hint as to a possible endogenous origin.