Murine macrophage cell lines and resident macrophages showed various levels of expression of the murine osteopontin (OP) gene, and macrophage stimulating agents were found to enhance transcription of the gene with kinetics which are unique for each stimulator. The organization of the murine OP gene was determined. The gene comprises six exons and five introns and spans approximately 4.8 kilobases. Exon 1 contains the 16 amino acids of the leader sequence. Exons 2, 3, 4, 5, and 6 encode 12, 27, 14, 94, and 129 amino acid residues, respectively. Exon 5 encodes regions containing 10 consecutive Asp amino acid residues and a Gly-Arg-Gly-Asp-Ser peptide. Exon 6 encodes the C-terminal half of OP and contains no 15- and 54-base pair nucleotide sequences which are deleted in murine OP cDNA compared to that of rat OP cDNA. Since Southern blot analysis indicated that the OP gene is a single copy, it is obvious that the murine OP cDNA has the sequence previously determined (Miyazaki, Y., Setoguchi, M., Yoshida, S., Higuchi, Y., Akizuki, S., and Yamamoto, S. (1989) Nucleic Acids Res. 17, 3298). A comparison with the cDNA sequences reported previously suggested the presence of nucleotide sequence polymorphisms. The 5‘ end of the murine OP gene was defined by primer extension and S1 nuclease mapping. Sequence analysis of the 5‘-flanking DNA revealed the presence of many potential regulatory motifs.