Six T-cell growth promoting factor (TCGF)-dependent T-cell cultures and phytohemagglutinin-stimulated peripheral blood lymphocytes from a patient with metatastic malignant melanoma were karyotyped. Three of the TCGF-dependent cultures were noncloned, started after in vitro sensitization against autologous malignant melanoma cells in tissue culture. A fourth was a subculture of one of these. The two remaining T-cell cultures were generated by in vitro sensitization of the same individual's lymphocytes against allogeneic Epstein-Barr virus-transformed B-cell lines. One of the cultures (AV7) and its subculture (AV/LDP39) showed a 17q+ anomaly in 100% of metaphases studied. All other TCGF-dependent T-cell cultures and fresh blood lymphocytes had normal karyotypes. The possible mode of origin of the abnormality and its significance are discussed.