The production and characterization of a monoclonal antibody (MoAb) designated Ber-H2, directed against a new epitope of the Ki-1 (CD30) antigen, are described. In comparison with the formerly reported Ki-1 MoAb whose reactivity with Hodgkin and Reed-Sternberg (H-RS) cells in frozen tissue sections is well-documented, the Ber-H2 MoAb showed new, important features: the labeling intensity of the Ber-H2 MoAb was much stronger, and the number of positively labeled cells was higher. Most important, however, was that the Ber-H2 MoAb could be applied in routinely processed, formaldehyde-fixed, paraffin-embedded tissue sections. Therefore, it was possible to investigate an unprecedented number of tumors received as frozen or formaldehyde-fixed material for expression of the CD30 antigen. Beside Hodgkin's disease, the Ber-H2 MoAb labeled a variable number of cells in lymphomatoid papulosis, peripheral T-cell lymphomas, and angoimmunoblastic lymphadenopathy. Among B-cell non-Hodgkin's lymphomas (NHLs), some cases containing large centroblast-like or immunoblast-like cells or displaying plasma- cellular differentiation were positive. This finding was in keeping with the reactivity of the Ber-H2 MoAb with activated B-cell blasts and a subpopulation of plasma cells in paraffin sections of normal lymphoid tissue. The diagnostic value of the Ber-H2 MoAb was most significant for a group of anaplastic large-cell (ALC) lymphomas (formerly frequently referred to as malignant histiocytosis or regressive atypical histiocytosis), of which more than 50 cases could be investigated, owing to applicability in paraffin sections. Although about one third of these ALC lymphomas did not express the leukocyte common (CD45) antigen, they were consistently reactive with the Ber-H2 MoAb in both frozen and paraffin-embedded tissue sections. Using the Ber-H2 MoAb, these Ki-1 lymphomas could be easily distinguished from other nonlymphoid anaplastic large-cell tumors.