The aim of this study was to purify an acidic α-glucan-protein complex from the fruiting bodies of Pleurotus sajor-caju by using the cell wall-degrading enzymes, xylanase and cellulase. The acidic glucan-protein complex was separated from a polysaccharide extract by using DEAE Toyopearl 650M anion-exchange and Sepharose CL-6B chromatography. Its homogeneity was ensured by high-performance size-exclusion chromatography and agarose gel electrophoresis. The acidic glucan-protein complex had a molecular weight of approximately 182 kDa. Fourier transform infrared spectroscopy of the acidic glucan-protein complex revealed an α-glycosidic bond and the typical characteristics of polysaccharides and proteins. The amino acid composition of the protein moiety was dominated by proline, glycine, glutamic acid and aspartic acid, indicating that the protein was highly flexible and had a negative charge. Atomic force microscopy proved that the acidic α-glucan-protein complex existed in a spherical conformation. The acidic α-glucan-protein complex stimulated the activation of macrophages, including the production of nitric oxide and tumor necrosis factor-α.