The ultrastructure and distribution of some viral and cellular antigens was examined in lymphoid C91/PL and fibroblastic HOS/PL cells infected with human T-cell leukaemia virus type I. Tubuloreticular structures in the endoplasmic reticulum, characteristic of virus-infected or interferon-treated cells, were found in both cell types. Virions were observed particularly in the HOS/PL cells in budding and mature forms and in coated pits, coated vesicles and lysosomes, indicative of receptor-mediated endocytosis. Using immunogold indirect labelling, viral antigens, recognized by sera of infected patients and by monoclonal antibody, were detected in patches on the cell surface and in material loosely attached to the envelope of some virions. Beta 2-microglobulin was associated with virion envelopes from C91/PL and HOS/PL cells, but major histocompatibility complex class 1 (HLA) antigens were not associated with virions even when produced by HLA-positive HOS/PL cells.