Protein kinase activity was studied in cytosolic extracts from leaves of wild type Arabidopsis thaliana, the ethylene-insensitive mutant, etr1, and the constitutive triple-response mutant, ctr1. Treatment of wild type with ethylene resulted in increased myelin basic protein (MBP) phosphorylation. In etr1, constitutive protein kinase activity was lower than in wild type, but in ctr1, activity was enhanced. A protein of M(r) approximately 47 kDa associated with MBP-phosphorylating activity was detected using in gel protein kinase assays and phosphorylation of this protein was promoted by ethylene treatment in wild type while activity in the mutants reflected that of MBP phosphorylation. Both MAPKinase (ERK 1) and phosphotyrosine antibodies immunoprecipitated MBP-phosphorylating activity and detected a polypeptide band at M(r) approximately 47 kDa. Immunoprecipitated MBP-phosphorylating activity was again much lower in etr1 compared to wild type but much higher in ctr1. Antibodies to phosphorylated MAPKinase recognised proteins at approximately 47 kDa and the signal was upregulated in response to ethylene. The data obtained suggest that the detected protein(s) is a MAPKinase and provide further evidence confirming that a MAPKinase cascade(s) is involved in ethylene signal transduction.