By three criteria, two biochemical and one immunochemical, the major postsynaptic density protein (mPSDp) is indistinguishable from the 50-kilodalton (kDa) alpha subunit of a brain calmodulin-dependent protein kinase. First, the two proteins comigrate on NaDodSO4/polyacrylamide gels. Second, iodinated tryptic peptide maps of the two are identical. Finally, a monoclonal antibody (6G9) that was raised against the protein kinase binds on immunoblots to a single 50 kDa band in crude brain homogenates and to both the alpha subunit of the purified kinase and the mPSDp from postsynaptic density fractions. The purified kinase holoenzyme also contains a 60-kDa subunit termed beta. A comparison of the peptide map of beta with the maps of 60-kDa proteins from the postsynaptic density fraction suggests that beta is present there but is not the only protein present in this molecular weight range. These results indicate that the calmodulin-dependent protein kinase is a major constituent of the postsynaptic density fraction and thus may be a component of type I postsynaptic densities.