Gene rearrangement and monoclonality have been detected in normal cells and in lymphoproliferative disease by using the polymerase chain reaction and primers for the V and J regions of the Ig heavy chain gene or T-cell receptor gamma-chain gene. Using the Ig primers monoclonality was detected in 20 of 20 normal B-lymphocyte clones and in 39 of 52 cases of various types of B-lymphoproliferative disease, but not in 11 cases of T-lymphoproliferative disease. Using the T-cell receptor primers, monoclonality was detected in 186 of 192 normal T-lymphocyte clones, in 11 of 11 cases of T-lymphoproliferative disease, in 9 of 12 cases of B-acute lymphocytic leukemia, and in 1 of 21 cases of B-non- Hodgkin's lymphoma, but not in nine cases of B-chronic lymphocytic leukemia nor in 10 cases of myeloma. Monoclonality was detected in material obtained by lymph node aspiration in four of six additional cases of non-Hodgkin's lymphoma. It was not detected in 10 cases of acute myeloid leukemia nor in four cases of reactive lymphadenopathy. Detection of gene rearrangement by the polymerase chain reaction has a number of advantages over Southern blotting and is likely to become the initial diagnostic technique of choice to detect monoclonality.