Normal rat kidney cells of the clone 49F and their Ki-MSV-transformed counterparts spontaneously release the same transforming growth factor (TGF) activity in an inactive form. By acidification followed by neutralization prior to assay, this TGF activity is unmasked and promotes anchorage-independent growth of the NRK-49F indicator cells in the presence of epidermal growth factor. The TGF activity released by both cell types has an apparent molecular weight of 9,000 under acidic conditions,

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... not compete for binding to epidermal growth factor receptors, is heat resistant but dithiothreitol and trypsin sensitive, and therefore is of the beta-TGF class.