Neuronal protein gene product 9.5 (PGP 9.5) most likely identical to ubiquitin carboxyl-terminal hydrolase isozyme L1 (UCH-L1) has been reported to be expressed almost exclusively in neuronal and neuroendocrine tissues. By two-dimensional (2D) immunoblotting, comigration and microsequencing of proteins recovered from 2D gels we have identified PGP 9.5 UCH-L1 as polypeptide IEF SSP 6104 (Mr = 27,000, pI = 5.49) in the comprehensive 2D gel cellular protein database of human embryonal lung MRC-5 fibroblasts [(1989) Electrophoresis 10, 76 115; (1990) Electrophoresis 11, 1072 1113]. This protein is expressed at high levels in quiescent and proliferating cultured normal fibroblasts and is strongly down-regulated (about 10 times) in their transformed counterparts.