The authors have examined cellular areas of lymphoma tissue in 28 cases of Hodgkin's disease (HD) or anaplastic large cell lymphoma (ALCL, 'Ki-1 cell lymphoma') to evaluate the boundaries between the two entities. Methods applied included conventional histology; test point analysis; semiautomated morphometry of nuclear profile features of Reed-Sternberg and other atypical large cells (RSALCs); and immunohistochemistry of these elements on all paraffin sections and, in 15 cases, on frozen sections. Mean nuclear profile morphotypes of RSALCs per case varied independently of immunophenotype and histologic diagnosis. Conversely, immunohistochemistry demonstrated significant, although not consistent, preferential positivities of these CD30+ elements for CD15 in HD, and for epithelial membrane antigen (EMA) and CD43 in ALCLs. In the latter, RSALCs also exhibited a tendency for CD45 and CD45RO positivity and for the expression of T-cell-associated antigens. However, there were considerable overlaps. This continuous spectrum of RSALC nuclear profile morphotypes and immunophenotypes, ranging from HD over questionable cases, intermediate between HD and ALCL, to ALCLs, was paralleled by differences in the reactive component of lymphomas. Lymphocytes and granulocytes were significantly deficient in ALCLs.