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We examined 91 specimens (from 87 patients) for the expression of B- cell- and T-cell-associated differentiation antigens and rearrangements of the Ig and beta-chain of the T-cell (beta-TCR) genes. Of these, 74 were representative of various histologic subtypes of non-Hodgkin's lymphoma and related disorders, 11 of Hodgkin's disease, and 6 of reactive lymphoid hyperplasia. An Ig gene clonal rearrangement correlated to a monotypic (kappa/lambda) phenotype in 32 of 33 histologically defined lymphoma samples. The genotypic analysis also confirmed clonality in six of seven malignant diffuse lymphomas that were nonmonotypic but expressed pan-B antigens; in four, more than one clone was detected within individual tumors. A beta-TCR clonal rearrangement was found in 19 of 19 tumor samples considered as malignant T-cell lymphoma on the basis of histopathology and of the CD3- positive phenotype of tumoral cells, and in two cases of CD3-positive lymphomatoid disorders. A loss of pan-T antigens (CD7, CD5, CD2, CD4/CD8) was observed in all but three of these CD3-positive samples. Such an incomplete T-cell phenotype always correlated to the presence of a monoclonal process as revealed by genotypic analysis. DNA analysis was the only way to demonstrate clonality in other samples with either a polymorphous (partial involvement, pseudolymphoma, angioimmunoblastic lymphodenopathy [AILD]) or an undifferentiated (large cell anaplastic) phenotype. It is concluded that although in the majority of cases immunophenotyping alone provides criteria adequate for the diagnosis of lymphoid malignancy, in some, particularly polymorphous or large cell anaplastic processes, genetic probe analysis was additionally discriminative.