Immunoreactivity with monoclonal antibodies against the intermediate filament protein, vimentin, and epithelial membrane antigen (EMA) was examined in 330 cases of lymphoma (317 non-Hodgkin's and 13 Hodgkin's lymphomas), 12 reactive lymph nodes and mononuclear cells of the peripheral blood using either indirect immunoperoxidase staining or the avidin-biotin immunoperoxidase complex technique. The cell origin of each tumor was established using a panel of monoclonal antibodies against lymphocyte differentiation antigens. There were 41 T-cell, 247 B-cell and 29 undetermined lymphomas, and 13 cases of Hodgkin's disease in the series. Vimentin was expressed in 24 T-cell lymphomas (58.5%) and 60 B-cell lymphomas (24.2%). This difference in frequency was statistically significant. Vimentin expression in follicular lymphomas was less frequent than in diffuse B-cell lymphomas. In diffuse lymphomas, small and medium cell types were more reactive with anti-vimentin than large cell types. Reed-Sternberg cells (R-S cells) in Hodgkin's disease were positive for vimentin in 11 cases (84.6%). The frequency of EMA reactivity in lymphomas was low, particularly in T-cell lymphomas. No positive cases were found among follicular lymphomas. In diffuse non-Hodgkin's lymphomas, EMA was expressed only in mixed and large cell types, but never in smaller ones. In conclusion, monoclonal antibodies against vimentin and EMA appear to be of limited usefulness for the diagnosis of non-Hodgkin's lymphomas, but anti-vimentin antibody may be used as an adjunct to the diagnosis of R-S cells in Hodgkin's disease.