The enzymes p70s6k and p85s6k are two isoforms of the same kinase and are important in mitogenesis. Both isoforms are activated by a complex phosphorylation event and lie on a common signalling pathway, distinct from that of the p42mapk/p44mapk kinases. Activation of p42mapk/p44mapk is triggered by sequential activation of the GDP-GTP exchange factor Sos, the GTP-binding protein p21ras, and protein kinases p74raf and p47mek (refs 7-10). As p21ras transformed cells have increased S6 phosphorylation, we tested whether the p70s6k/p85s6k signalling pathway bifurcates between p21ras and p42mapk/p44mapk. We found that mutants of p74raf and p21ras blocked activation of epitope-tagged p44mapk but not epitope-tagged p70s6k. Moreover, in cells expressing human platelet-derived growth factor receptors lacking the kinase-insert domain, the growth factor activates p21ras but not p70s6k/p85s6k. The critical autophosphorylation site for p70s6k/p85s6k activation within this domain is a tyrosine at residue 751. Our results show that the p70s6k/p85s6k signalling pathway is independent of p21ras, that it bifurcates from the p21ras pathway at the receptor, and that it is initiated by autophosphorylation at a specific site.