A PCR-based method that determines VDJ junction size patterns in 24 human TCR V beta subfamilies was used to analyze T cells infiltrating sequential malignant melanoma biopsies for the presence of clonal expansions. Infiltrating T cell populations were found to present clonal expansions over a more or less complex polyclonal background. Two clones from a single patient were sequenced and detected in three different tumor sites (skin biopsies), whereas only one of them was also present in peripheral blood. Biopsies from this patient did not show major repertoire changes during in vivo IL-2 treatment. In contrast, in biopsies from a second patient, the expression of all the detected V beta subfamilies was increased and a larger number of clones expanded, probably as a result of therapy. A similar evolution was found among infiltrating T cells cultured in vitro from a third patient for several weeks in the presence of IL-2, where the largely polyclonal repertoire of fresh T cells (from invaded lymph nodes) was dramatically reduced to mainly clonal expansions in all V beta subfamilies detected. The high resolution method used here enables a rapid, comprehensive, qualitative, and semiquantitative description of the T cell repertoire of heterogeneous cell populations. Its use in conjunction with a functional analysis of clones detected within these populations should provide a better understanding of the evolution of the T cell repertoire among tumor-infiltrating lymphocytes during the progression of the disease and as a response to immunotherapy.