The aberrant expression of a plasminogen activator (PA) by Moloney virus (MuLV)-transformed mouse lymphocytes and its relation to cell phenotype and tumor growth have been studied. Nine cultured cell lines were established from neoplastic splenic and thymic tissues obtained from B10 congeneic mice inoculated with MuLV and killed when overtly leukemic. Cell surface markers were assayed by microcytotoxicity tests, the concentration of MuLV p30 and group-specific MuLV gp 70 was determined by radioimmunoassays and the expression of PA activity was assessed in a fibrin-agar plate method. PA activity of 24 h serum-free culture supernatant, intact cells or cell lysates (2 X 10(5) cells/ml) was expressed in International Units by reference to a urokinase standard curve. Tumor extension and cell morphology were investigated by histologic and cytomorphologic analysis. In all cases the cell lines were derived from T cells. PA activity is not expressed by normal lymphocytes, but variations in PA expression were observed in the transformed cells. Five out of nine transformed cell lines showed PA activity with a range of 1.3 to 9.9 IU/ml. No PA activity could be detected in the other cell lines. No correlation was found between PA expression and the cell-surface-expressed phenotype, neither was there any correlation between the PA content, the cytopathological features and the degree and type of organ infiltration. This lack of correlation indicates that there is no relation between PA activity and the expression of the transformed phenotype, and that the presence of PA activity seems to be irrelevant to the tumorigenic capacities of the transformed cell lines.