Rous-sarcoma-virus-transformed fibroblasts having low levels of plasminogen activator

Several clones of transformed chick embryo fibroblasts infected with wild-type B77-C or Prague-C strain of Rous sarcoma virus have been isolated from soft agar suspension. These clones were screened for plasminogen activator activity by overlaying monolayer cultures with medium containing agar, casein, and chicken plasminogen. Twenty-three percent of all of the isolated clones showed little caseinolytic activity, 42% had intermediate activity, and 35% had high activity. Although the clones with low plasminogen activator activity had no more than twice the activity shown by uninfected fibroblasts, they did not differ significantly from clones possessing high levels of plasminogen activator in their morphology, 2-deoxyglucose transport, or efficiency of colony formation in soft-agar.