The most important decision in transfusion medicine is whether a transfusion is truly needed. The second most important decision is what blood component is to be given. Administration of blood components is usually less risky than administration of whole blood. Careful attention to correct technique for collecting, preparing, storing, and administering blood products can help prevent many transfusion reactions. Patients should be carefully monitored while they are receiving a transfusion. Most transfusion reactions are dose-dependent; early recognition can avert disaster. Diphenhydramine (0.5 mg/kg) helps prevent hypersensitivity reactions. Glucocorticoids do not help prevent hypersensitivity reactions. Steroids should not be given unless they are necessary to treat the underlying disease condition or to treat shock. Part I of this two-part presentation discusses the causes and consequences of canine transfusion reactions. This article describes the technique for a major crossmatch, which should be performed for all dogs receiving erythrocytes - even from universal donors.