cAPK has provided many insights into the functioning of the diverse family of eukaryotic protein kinases. The fact that a particular amino acid in the catalytic core is conserved is an indication that the residue plays an important role; however, questions concerning function remain obscure. With the catalytic subunit, the assignment of amino acids that participate in catalysis has begun, and in many instances that function appears to be conserved in the other protein kinases. Although the regulatory subunit and the use of cAMP to release its inhibitor effects is unique to cAPK, the general mechanism of a small autoinhibitory region occupying the peptide binding site and thus preventing access of other substrates may be invoked frequently by other protein kinases. Coupling recombinant approaches with protein chemistry is allowing us to decipher at least some of the molecular events associated with cAMP-binding and holoenzyme activation. Although the next chapter in the history of cAPK will undoubtedly include three-dimensional structures, the chemical information remains as an essential complement for interpreting those structures and eventually understanding the molecular events associated with catalysis and activation.