Growth factor-induced signalling leads to activation of members of the Ras family and subsequent stimulation of different Raf isoforms. Within the mechanism of Raf activation, two isoforms of Raf, cRaf and BRaf, may cooperate. We investigated the relationship between cRaf and BRaf and found that active Ras induced heterodimerization of cRaf and BRaf, an effect that was dependent on the serine residue at position 621 of cRAF: Moreover, we also found that cRaf COOH-terminus constitutively associated with BRaf, whereas the NH(2) terminus did not, even in the presence of active RAS: These data suggest that Ras induces the cRaf-BRaf complex formation through the exposure of 14-3-3 binding sites in the COOH-terminus of cRAF: Thus, Ras-induced cRaf-Braf heterodimerization may explain the observed cooperativity of cRaf and BRaf in cells responding to growth factor signals.