The techniques developed in this investigation for the analysis of the structure and properties of immobilized alkaline phosphatase subunits can be applied to the characterization of other immobilized enzymes. The methods for determining the quaternary structure of immobilized subunits and of detecting contaminating oligomeric structures are important for the clear interpretation of the properties of subunit preparations. Complementation of immobilized subunits with labeled soluble subunits can be directly monitored by measuring isotope ratios; moreover, titrations of this type can be used to form specific hybrid oligomers which could not be prepared as a homogeneous population in solution. The use of immobilized protein derivatives makes it possible to obtain stable subunit preparations, and thus study the properties of single subunits. Even the rates of "inactivation" of subunits without catalytic activity can be quantitated after titration with soluble subunits. The availability of these methods for studying immobilized enzymes, combined with the advantages of using these derivatives, should increase the general potential of immobilization as a technique for studying oligomeric proteins.