The plasminogen activator (PA) activity produced by Syrian hamster embryo (SHE) cells in different stages of neoplastic conversion was analysed. PA activity was characterized immunologically and by SDS-PAGE. Normal SHE cells had a very low PA activity. Although activity of either the tissue type of PA (t-PA) or the urokinase type (u-PA) or both were found to be increased in most immortal or transformed SHE cells, there was no correlation between enhanced production of a particular PA type and the development of the immortal or transformed phenotype. However, within a group of cell lines clonally derived from a culture of immortal cells, a positive correlation was found between extracellular t-PA, but not u-PA, activity and cellular growth rate. For the Syrian hamster PA species, crossreacting with anti-human u-PA, a mol. wt of 39 kd was observed. For the Syrian hamster PA species, crossreacting with anti-human t-PA, multiple species were found with mol. wts of 98, 72 and 59 kd respectively. Evidence was obtained that the 72-kd species represents the intact enzyme, the 59-kd species a partial digestion product thereof and the 98 kd species, which often appears as a doublet, a complex of either of these species with an inhibitor, likely to be secreted by the same cells. Finally, our data suggest a novel mechanism for the enhancement of t-PA activity of transformed cells, namely by a decrease in the effective extracellular amount of putative inhibitor.