Three alpha 1-acid glycoprotein (AGP) cDNA clones have been isolated from a mouse liver library. Restriction enzyme mapping and sequencing of these cDNAs have shown that two, pMAGP2 and pMAGP3, are virtually identical, whereas the third, pMAGP4, differs significantly both in sequence and restriction enzyme sites. The sequences are 91% identical and differ from each other by single base differences exclusively. No frameshifts are observed, and no termination codons are generated by the single base differences. We interpret these data to indicate that there are at least two distinct AGP genes in the mouse and that two species of AGP mRNA are formed by the transcription of these genes in the liver. Based on the large number of single amino acid substitutions previously observed in human AGP (Schmid, K., Kaufmann, H., Isemura, S., Bauer, F., Emura, J., Motoyama, T., Ishiguro, M., and Nanno, S. (1973) Biochemistry 12, 2711-2724), we propose that at least two functional AGP genes may also exist in humans.