A 12-yr-old female chimpanzee (Pan troglodytes) was presented as severely emaciated and with generalized muscle weakness. Hematology and biochemistry revealed severe anemia and hypokalemia. The chimpanzee was treated supportively and symptomatically; although initially stable, the animal deteriorated rapidly on day 5, becoming depressed and jaundiced with further deterioration of anemia. To address the decline, a prompt transfusion of compatible and cross-matched fresh whole blood from a healthy adult male chimpanzee was administered over 120 min. During transfusion, an immediate reduction in the recipient's tachycardia was noted and substantial clinical improvement continued over 24 hr posttransfusion; no adverse transfusion reactions were observed.