A genomic clone coding for the alpha subunit of the mouse complement receptor type 3 and the cellular adhesion molecule Mac-1 has been isolated directly from a genomic library using synthetic oligonucleotide probes based on the amino-terminal amino acid sequence of the protein. The identity of the clone has been established by DNA sequencing and in vitro translation of hybrid-selected mRNA. The gene is present in a single copy in the murine genome. The region containing the amino-terminal exon has been sequenced. RNA gel blotting shows that the Mac-1 alpha-subunit mRNA is 6 kilobases in length. Mac-1 alpha-subunit mRNA is present in macrophages but not T lymphoma or L cells. During gamma interferon-stimulated maturation of the mouse premyelocytic cell line M1, Mac-1 alpha-subunit mRNA is induced. This corresponds with the tissue distribution of the Mac-1 alpha subunit, showing expression is regulated at least partially at the message level.