The synthesis and circulating concentrations of acute phase proteins (APPs) are regulated in response to inflammation, infection, trauma, and neoplasia in many domestic and nondomestic species. The APP response is species specific; thus, assays must be validated, and reference intervals must be determined for each species. Koalas (Phascolarctos cinereus) are a vulnerable species, threatened by infectious and inflammatory diseases both under human care and in the wild. The ability to diagnose, treat, and provide prognosis for common koala health problems is challenged by the paucity of sensitive diagnostic tests. Assays for C-reactive protein, serum amyloid A, and haptoglobin were validated for use in koalas. Reference intervals were established using the robust method recommended by the American Society for Veterinary Clinical Pathology based on serum samples from 26 healthy koalas at the San Diego Zoo. The reference intervals are as follows: C-reactive protein, 3.2–24.1 mg/L; serum amyloid A, 0.10–0.45 mg/L; haptoglobin, 0.10–0.64 mg/ml.