Loss of heterozygosity (LOH) of distal chromosome 4 was studied to localize a putative tumor suppressor gene in mouse lung neoplasia. Previous studies have implicated this region and its homologue on human chromosome 1p36-34 as sites of potential tumor suppressor genes. One hundred adenocarcinomas and 38 adenomas of the mouse lung were examined for LOH using nine simple sequence length polymorphism markers in a polymerase chain reaction-based approach. Forty-four adenocarcinomas displayed allele loss, all of which included losses at marker D4MIT54, which defined a critical region of 3 centiMorgans as the likely location of a putative tumor suppressor gene. In 58% of the tumors displaying LOH, all markers used in this study incurred allele loss. In contrast, retention of heterozygosity was observed at all markers tested in each of the adenomas studied, which suggests that the inactivation of this tumor suppressor gene participates in mouse lung tumor progression.