Following our recent reports of detecting clonal immunoglobulin and T-cell receptor gene rearrangements by the polymerase chain reaction, we have improved and simplified the technique for use in diagnostic histopathology laboratories and determined, on coded samples, the sensitivity, specificity, and reproducibility of the modified methodology in distinguishing malignant lymphoma from reactive lymphoid hyperplasia and nonlymphoid tumors. Using only three primer pairs for the immunoglobulin heavy chain and T-cell receptor beta and gamma chain genes on well-characterized lesions of widely varying morphology and immunophenotype, clonal rearrangements were detected in 65% of B-cell lymphomas, and 77-82% of T-cell tumors. Specificity and observer consistency ranged from 93-97%. The method requires very careful control, particularly to avoid misinterpretation of results because of contamination and nonspecific amplification, but in its present form is relatively simple and inexpensive, and gives results on single paraffin-embedded sections within 24 h.