Rapid induction of 190s transcription by serum and phorbol esters requires the serum response element (ME). The SRE contains a 20 bp element of interrupted dyed symmetry (DSE) that is bound by p67/SRF. We have identified a hitherto unrecognized protein with an apparent size of 62 kd. This novel component,~ 62, is shown to be an integral but physically separable part of a ternary complex formed with p67/SRF and the SRE. Alone, p62 fails to bind the SRE but requires DSE-bound p67/SRF and sequences both within and outside the DSE for its interaction with DNA. In vivo, the response of the c-fos promoter to serum is severely impaired by mutations that abolish ternary complex formation in vitro.