The cyclic nucleotide PDE activity in crude tissue extracts is due to the composite activity of several distinct isozymes, each having different kinetic and functional characteristics. Most of these isozymes will hydrolyze the same substrates, that is, cyclic AMP and cyclic GMP. Therefore, it is difficult to conclusively identify and quantitate any given isozyme in crude systems. Since the cyclic nucleotide PDE are present in low concentrations in cells, it is also difficult to isolate these proteins without having specific solid phase affinity probes. This chapter described the use of monoclonal antibodies as probes to specifically identify, measure, characterize, and isolate the PDE isozymes in impure preparations. Monoclonal antibodies have been produced to the cyclic-GMP-stimulated PDE, ROS PDE, and a calcium-calmodulin-dependent PDE of bovine tissues. A polyclonal antiserum has also been produced to the cyclic-GMP-stimulated PDE. Sensitive immunoprecipitation assays for the measurement of PDE activity and cyclic GMP binding have been developed that have allowed the detection of femtomole amounts of each specific PDE isozyme. None of the antibodies appear to recognize antigenic determinants on other isozymes, suggesting that the PDE are antigenically distinct and therefore different proteins. Most of the monoclonal antibodies do not appear to affect kinetic parameters of the enzymes and have been used to specifically identify, measure, and characterize each isozyme in crude systems. A monoclonal antibody to the calcium-calmodulin-dependent PDE requires calcium and calmodulin for high-affinity binding to the enzyme. This apparent conformational requirement has been used to purify the isozyme from both bovine brain and heart. These two tissues contain enzymes with differing subunit MW on SDS-polyacrylamide gel electrophoresis, although no other biochemical differences were observed.