We cloned a previously characterized glioblastoma-derived parent cell line (12-18) in order to obtain a relatively homogenous population of human neural cells of neoplastic origin. These cells reach high densities in culture (over 100,000 cells/cm2) and have a high mean DNA content per cell of 18.1 +/- 0.9 pg. A histogram of the cloned cells' chromosome numbers revealed one peak and a modal near diploid number of 52, whereas the parent cell line had expressed polyploidy, with several peaks (including 52) at population doubling level 16. Several consistent results were obtained by Giemsa staining. A persistent structural alteration was the duplication of the long arm of chromosome #9 on to another arm of #9, and the translocation of the short arm of #9 to chromosome #21. We further observed that these cloned cells secrete a specific protease, a plasminogen activator (PA), into serum-free medium (SFM). This enzyme was assayed by the conversion of purified plasminogen to plasmin and the subsequent degradation by plasmin of 125I-labelled fibrin. Glioblastoma-derived cells had higher levels of cell-associated PA activity (2.9-fold) and released more PA activity into SFM (22-fold) than human fetal neural cells. The presence of this protease suggests a mechanism for the invasive character of these neoplasms (glioblastoma multiforme) in vivo.