Several human tumour cell lines contain genes that can transform NIH 3T3 cells into malignant cells. Certain genes have been classified as members of the ras oncogene family, namely, Ha-ras, Ki-ras or N-ras. The proteins encoded by the ras family are generally small (Ha-ras, for example, encodes a protein of molecular weight 21,000 named p21), and are associated with the inner surface of the plasma membrane. The only known biochemical property common to all forms of the ras proteins is the ability to bind guanine nucleotides, a property which may be closely related to the transforming ability of ras proteins. A GTP-dependent, apparent autophosphorylation (on threonine 59) activity has been identified only in the case of the v-Ha-ras protein. Although the role of these biochemical activities in the transformation process remains unclear, we have initiated studies to determine the possible biochemical interactions of ras proteins with other membrane components. We report here the evidence that epidermal growth factor enhances the guanine nucleotide binding activity of activated c-Ha-ras or v-Ha-ras p21, and phosphorylation of v-Ha-ras p21, suggesting that some mitogenic growth factors may regulate those activities.