This methods chapter elaborates on how a direct enzyme-linked immunosorbent assay (ELISA) is used to specifically detect and quantify murine alpha-1 antitrypsin (AAT). As a direct ELISA, it lacks some sensitivity as compared to the "sandwich" ELISA method; however, it does reliably differentiate between samples with varying amounts of the mouse AAT protein. This protocol relies on the principle of adsorption to coat each well with sera proteins, whereas detection occurs specifically using a two-step antibody combination. This procedure effectively identifies and quantifies murine AAT from a wide variety of samples including mouse serum, cell culture medium, and cell or tissue lysate.