Blood transfusions have been implicated in predisposing patients to infection by inducing immunosuppression. This study evaluated the effects of syngeneic (ST) and allogeneic (AT) blood transfusion with and without hemorrhagic shock (HS) to determine whether transfusion or the accompanying hemorrhage affected certain components of the immune response. Lewis rats received ST or AT at 10%, 20%, or 30% of blood volume. Hemorrhagic shock was induced in other animals, which were resuscitated with either shed blood or substituted 10%, 20%, or 30% ST or AT. Intradermal staphylococcal abscess size, peritoneal leukocyte elicitation, and peritoneal macrophage Ia receptor expression were selected to measure the immune system response. Hemorrhagic shock increased abscess size significantly (p less than 0.05), but ST or AT alone or in combination with HS had no effect. Both shock and transfusion per se increased macrophage Ia receptor expression (p less than 0.05), but no additive or synergistic effect was observed. Peritoneal leukocyte elicitation was not affected by HS, ST, or AT. These results suggest that HS and not blood transfusion is a major determinant of the risk of infection.