A deletion involving chromosome 3p (14-23) characteristically occurs in small cell lung cancer (SCLC). Reduction to homozygosity, rather than complete loss, is typically observed for genes in the deleted region. Lack of expression for genes encoded by this region, implying inactivation of all alleles, has not been previously described. We have examined the expression of aminoacylase-1 (ACY-1), encoded by chromosome 3p21, using both an electrophoretic activity assay and a monoclonal antibody-based ELISA. A variety of human tissues, including lung, brain, liver, kidney, heart, adrenal medulla, and erythrocytes have previously been tested for ACY-1 activity and antigen; all but erythrocytes are positive. Thus, ACY-1 is expressed in all nucleated human cells examined to date. ACY-1 was undetectable in a significant number of SCLC cell lines (4/29) and tumors (1/8), but not in non-small cell lung cancer (NSCLC) cell lines (0/19) or tumors (0/9), nor in a variety of other human cell lines (0/15) or colon tumors (0/8). In addition, reduced (approximately 10% of normal) ACY-1 expression was common in SCLC cell lines (14/29) and tumors (3/8), but not in NSCLC cell lines (1/19) or tumors (0/9), nor in other human cell lines (0/15) or colon tumors (0/8). Thus, low or undetectable ACY-1 expression is highly specific for SCLC and occurs in both cell lines and tumor tissue. The finding of undetectable ACY-1 expression in SCLC supports the hypothesis that inactivation of all alleles of specific chromosome 3p genes occurs in a SCLC in a fashion analogous to Rb gene inactivation in retinoblastoma, and suggests that the structural gene for ACY-1 may be closely linked to a putative SCLC tumor suppressor gene.