We have reinvestigated the recent proposal that the multicatalytic proteinase, together with other components of reticulocyte lysate, may become incorporated into a very large, "26 S" proteinase complex via an ATP-dependent process. Different from these published results, we consistently isolate the multicatalytic proteinase as a 650,000 Da "20 S" multisubunit proteinase. Analysis on nondenaturing polyacrylamide gels of reticulocyte fractions containing the putative complexed form of the multicatalytic proteinase reveal that activity against succinyl-Leu-Leu-Val-Tyr-7-amino-4-methylcoumarin is associated with two groups of protein of different molecular mass. One migrates like multicatalytic proteinase purified to homogeneity, displays, on sodium dodecyl sulfate gels, a set of protein species in the range of 23,000-32,000 Da, characteristic of the multicatalytic proteinase, and is recognized by a monospecific antibody to the enzyme. In contrast, the activity associated with the higher molecular mass (26 S) proteinase complex lacks the typical multicatalytic proteinase subunits and is devoid of antigenic material, when tested with the antibody. These results confirm and extend our recent findings in mouse liver by showing that the multicatalytic proteinase is not a constituent of a 26 S proteinase complex.