The subunit patterns of the proteasomes, that were purified from muscle, liver and brain, were found to be significantly different from one another. Furthermore, the proteasomes from adult and embryonic tissues of the same types also differed from each other in their subunit patterns. In addition, the specific activities of the purified proteasomes for peptide-cleavage, but not for casein-hydrolysis, appeared to be varied among the enzymes isolated from the different tissues. Thus, expression of a large number of proteasome subunits appears to be tissue-specific and under developmental control, although its relation with the multicatalytic activities of the proteasomes remains unclear.