We have previously shown that chick muscle extracts contain at least 10 different ubiquitin C-terminal hydrolases (UCHs). In the present studies, one of the enzymes, called UCH-1 was partially purified by conventional chromatographic procedures using ¹²⁵I-labeled ubiqui-tin-αNH-MHISPPEPESEEEEEHYC as a substrate. The purified enzyme behaved as a 35-kDa protein under both denaturing and nondenaturing conditions, suggesting that it consisted of a single polypeptide chain. It was maximally active at pHs between 8 and 9, but showed little or no activity at pH below 6 and above 11. Like other UCHs, its activity was strongly inhibited by sulfhydryl blocking reagents, such as iodoacetamide, and by ubiquitin-aldehyde. In addition to Ub-PESTc, UCH-1 hydrolyzed ubiquitin- αNH-protein extensions, including ubiquitin- αNH-carboxyl extension protein of 80 amino acids, ubiquitin-αNH-dihydrofolate reductase, and poly-His-tagged di-ubiquitin. This enzyme was also capable of generating free ubiquitin from mono-ubiquitin- εNH-protein conju-gates and from branched poly-ubiquitin chains that are ligated to proteins through εNH-isopeptide bonds. These results suggest that UCH-1 may play an important role in the generation of free ubiquitin from ubiquitin-ribosomal protein fusions and linear poly-ubiquitin, as well as in recycling of Ub molecules after degradation of poly-ubiquitinated protein conjugates by the 26S proteasome.