Selected lymphocytes from a patient with rheumatoid arthritis were infected with Epstein-Barr virus (EBV) and the emerging cell line (RF-AN) has now been successfully maintained in culture for more than two years. Multiparameter studies, including surface markers, ultra-structure and scanning immuno-electron microscopy, were utilized to characterize and evaluate this unique human cell line. Like other B-lymphoblastoid cell lines, RF-AN cells display multiple microvilli, contain surface and intracytoplasmic immunoglobulin (IgM lambda) and continuously produce monoclonal IgM lambda in vitro. The latter serves as an autoimmune antibody directed against IgG and is termed rheumatoid factor (RF). When RF-AN cells were incubated with human or rabbit IgG conjugated to polystyrene (latex) microspheres, almost all cells were labeled with the marker. However, cells did not label with goat IgG conjugated to latex. RF-AN cells were also labeled with latex microspheres coupled to goat antisera directed against human IgM or lambda chains but not with goat anti-human IgG or anti-human kappa chains. It is suggested that this mode of immunomicroscopy is reliable and may provide more useful data than other standard immunological techniques.