Formalin-fixed, paraffin-embedded tissue from B-cell malignant lymphomas (26), reactive lymphadenopathies (8), non-B-cell malignancies (5), and atypical lymphoproliferative lesions (7) were analyzed for clonal immunoglobulin heavy chain gene rearrangement by the polymerase chain reaction (PCR), using consensus primers for the variable and joining regions of the gene. By employing a high-resolution gel electrophoresis technique, we were able to demonstrate one or two dominant bands, indicating a clonal population, in 15 of the 23 cases (65%) of B-cell lymphoma in which amplification occurred. Six of six reactive lymph nodes in which amplification occurred produced a multi-banded pattern indicative of a polyclonal population. This improved PCR technique allows a clearer distinction between clonal and polyclonal patterns than other previously proposed methods. It also works well in paraffin-embedded tissue and may therefore be a useful adjunct to the diagnostic armamentarium applied to archival material.