A sensitive method of screening for dominant T cell clones in small samples of DNA has been developed using the polymerase chain reaction to amplify and identify T cell gamma receptor gene rearrangements. It can detect such rearrangements in nanogram quantities of DNA from cultured T cell clones, even in the presence of 20-100 parts of polyclonal lymph node DNA, and works with DNA extracted from paraffin sections of cloned T cells which have been fixed in formalin. Presumptive clonal reactions have been obtained in preliminary tests on 8 of 10 unfixed T cell lymphomas but in 0 of 10 reactive lymph nodes.