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Quail embryo fibroblasts infected with any of the four natural avian myc gene‐containing virus strains (MC29, CMII, OK10 and MH2) or with the myb, ets‐containing E26 acute leukemia virus, were examined for their expression of several transformation‐associated parameters. All myc‐containing viruses, but not E26 or Rous sarcoma virus (used as a control) induced a dramatic stimulation of cell proliferation. In addition, the myc virus‐transformed cells exhibited prominent nucleoli, possibly as a consequence of their increased proliferation. Cells transformed by MC29, OK10, MH2 and E26 were capable of growing in semi‐solid medium and showed a loss of actin cables and, in most cases, of an ordered fibronectin distribution. All of the myc virus‐transformed fibroblasts, as well as the E26‐transformed cells, were unable to form tumors in nude mice, indicating that the myc gene (and the myb/ets genes) are not sufficient for the induction of a fully malignant phenotype in avian fibroblasts.