Protocol describes how tagged Mos kinase is translated in vitro, immunopurified, and used in a kinase assay. Protocol includes: Translation of Xenopus Mos Kinase; Antibody to Antigen Binding; Protein A Sepharose to Antibody Binding; Kinase Reactions on Immunoprecipitated Material; Polyacrylamide Gel Analysis of Immunoprecipitates. Includes protocol hints.