A synthetic inhibitor of the mitogen-activated protein kinase (MAPK) cascade, PD 098059, has been identified. This compound selectively inhibits the MAPK/ERK kinase (MEK), which is essential for activating MAPK. PD 098059 prevents MAPK activation and subsequent phosphorylation of its substrates in both in vitro and in vivo settings. It also inhibits cell growth stimulation and reverses the transformed phenotype of ras-transformed cells. These findings indicate that the MAPK pathway is crucial for cell growth and maintenance of the ras-transformed phenotype. PD 098059 is a valuable tool for studying the role of the MAPK cascade in various biological contexts. The MAPK pathway is activated by diverse extracellular stimuli, including growth factors, hormones, and stress. MAPKs are activated by phosphorylation on threonine and tyrosine residues, and their activation is catalyzed by MEKs. MEKs are activated by upstream kinases, including those related to the ras protooncogene. PD 098059 inhibits MEK activity without affecting MAPK itself, as demonstrated by in vitro kinase assays and immunoblot analysis. The compound is reversible and does not inhibit other kinases such as Raf, cAMP-dependent kinase, or protein kinase C. PD 098059 inhibits MAPK activity in cultured cells, reducing tyrosine phosphorylation and thymidine incorporation, which are indicators of MAPK activation. It also reverses the transformed phenotype of ras-transformed cells, suggesting that the MAPK pathway is essential for maintaining the transformed state. PD 098059 acts in a cytostatic manner, inhibiting cell growth without causing cell death. It also alters the morphology of ras-transformed cells, indicating its role in reversing the transformed phenotype. These results demonstrate that PD 098059 is a selective inhibitor of MEK and the MAPK cascade, providing a valuable tool for studying the role of this pathway in various biological processes.A synthetic inhibitor of the mitogen-activated protein kinase (MAPK) cascade, PD 098059, has been identified. This compound selectively inhibits the MAPK/ERK kinase (MEK), which is essential for activating MAPK. PD 098059 prevents MAPK activation and subsequent phosphorylation of its substrates in both in vitro and in vivo settings. It also inhibits cell growth stimulation and reverses the transformed phenotype of ras-transformed cells. These findings indicate that the MAPK pathway is crucial for cell growth and maintenance of the ras-transformed phenotype. PD 098059 is a valuable tool for studying the role of the MAPK cascade in various biological contexts. The MAPK pathway is activated by diverse extracellular stimuli, including growth factors, hormones, and stress. MAPKs are activated by phosphorylation on threonine and tyrosine residues, and their activation is catalyzed by MEKs. MEKs are activated by upstream kinases, including those related to the ras protooncogene. PD 098059 inhibits MEK activity without affecting MAPK itself, as demonstrated by in vitro kinase assays and immunoblot analysis. The compound is reversible and does not inhibit other kinases such as Raf, cAMP-dependent kinase, or protein kinase C. PD 098059 inhibits MAPK activity in cultured cells, reducing tyrosine phosphorylation and thymidine incorporation, which are indicators of MAPK activation. It also reverses the transformed phenotype of ras-transformed cells, suggesting that the MAPK pathway is essential for maintaining the transformed state. PD 098059 acts in a cytostatic manner, inhibiting cell growth without causing cell death. It also alters the morphology of ras-transformed cells, indicating its role in reversing the transformed phenotype. These results demonstrate that PD 098059 is a selective inhibitor of MEK and the MAPK cascade, providing a valuable tool for studying the role of this pathway in various biological processes.