We have previously isolated, from agar suspension culture, clones of chicken embryo fibroblasts transformed by B77 and Prague strains of Rous sarcoma virus (RSV) that varied in the expression of plasminogen activator activity [Wolf, B. A. & Goldberg, A. (1976) Proc. Natl. Acad. Sci. USA 73, 3613-3617]. All of the clones exhibited an altered cellular morphology, an increased rate of sugar transport, and a high efficiency of colony formation in agar suspension regardless of the level of plasminogen activator. Because B77 and Prague strains of RSV replicate as well as cause sarcomas in chickens, the tumorigenicity of the transformed cells could not be evaluated with clones of these cells. In order to determine the oncogenicity of clones with various levels of plasminogen activator, it was necessary to isolate cells transformed by the replication-defective Bryan strain of RSV, which release noninfectious virus. All of the agar suspension clones of transformed cells, derived by infection of chicken embryo cells with replication-defective Bryan RSV, fell within the continuum observed for B77- and Prague-transformed clones with respect to altered morphology, increased rate of sugar transport, efficiency of colony formation in agar suspension, and variations in plasminogen activator activity. All of the clones, regardless of the level of plasminogen activator, produced tumors when as few as 5 x 10(2) cells were injected into the wing web of 1-day-old chicks. The latency period for tumor formation after injection of cells was similar regardless of the level of plasminogen activator of the injected cell. Primary explants of tumors resulting from inoculation of clones having low, intermediate, or high activator activity displayed a spectrum of activator activity.