Reactive immunoblastic proliferations may be confused with certain types of non-Hodgkin's lymphoma and Hodgkin's disease on morphologic grounds. In addition, a characteristic antigen, CD30 (Ki-1; BerH2) on these neoplastic entities may be observed in morphologically atypical yet reactive florid immunoblastic proliferations such as those associated with acute infectious mononucleosis. Although it has been documented, a large series determining the frequency and extent of CD30 antigen expression on a variety of nonneoplastic immunoblastic proliferations is lacking. The authors studied 14 florid immunoblastic proliferations (9 in lymph nodes and 5 in tonsils) for CD30 antigen expression and for B- and T-cell paraffin markers. In situ hybridization to determine the presence of Epstein-Barr virus (EBV) genomes also was performed. Cases were classified into monospot-positive acute infectious mononucleosis (4 cases), EBV-related lymphoproliferative disorder suggestive of acute infectious mononucleosis (5 cases), and other etiologies (5 cases). CD30 Antigen expression was found on the immunoblasts in cases from all three categories and overall in 9 (64%) of 14 specimens. CD30 reactivity in the positive cases varied from occasional to numerous positive cells; 4 samples (3 EBV-related lymphoid proliferations and 1 vaccine-related lymphadenopathy) had numerous CD30-reactive immunoblasts. Expression of CD30 antigen on B or T cells and prominence of B or T cells within a proliferation were variable. Significant "atypia" of immunoblasts was found only in EBV-related disorders and correlated with B-cell prominence of the infiltrate. Appropriate clinical correlation and ancillary laboratory data are necessary to assist in differentiating these CD30(+)-reactive disorders from similar-appearing malignant lymphomas. Most important, a fresh tissue sample should be procured and adequately processed to allow for comprehensive determination of clonality and cellular lineage.