It has been suggested that a key event in growth factor-induced p21Ras activation by the guanine nucleotide exchange factor Sos, is the recruitment of Sos to the plasma membrane by its interaction with the adaptor protein Grb2. However, other evidence argues that the sub cellular localisation of Sos is independent of Grb2, and that the Sos/Grb2 interaction can be dispensed with for p21Ras activation. To clarify the role of the Sos/Grb2 interaction in ligand-stimulated p21Ras activation, we have utilised the observation that overexpression of the Sos C-terminal domain can effectively inhibit p21Ras-dependent signalling in three different mammalian systems. We have shown that concurrent expression of Grb2, but not SH2 or SH3 domain mutants of Grb2, or the alternative adaptor protein Nck, can rescue this inhibitory effect of the C-terminus. This shows that the Grb2/Sos interaction is required to mediate growth factor-dependent activation of p21Ras, and requires the presence of intact SH2 and SH3 domains of Grb2. This approach was also used for a functional analysis of Sos which revealed that growth factor dependent signals are transmitted through both the N-terminal and C-terminal domains.