Avian influenza virus (AIV) of waterfowl origin, A/Mallard/Ohio/184/86 (H5N1), was used to evaluate the effect of AIV infection on the functional capabilities of the immune system in mallard ducks. The three main arms of the immune system--humoral, cell-mediated, and cellular--were evaluated. The integrity of the humoral immune system after AIV infection was evaluated by measuring total immunoglobulin and IgG antibody production to sheep erythrocytes and Brucella abortus antigen using hemagglutination and microagglutination assays, respectively. Cell-mediated immunity was evaluated using mitogen/antigen stimulation assays, and by measuring the cutaneous basophilic hypersensitivity response to intradermal phytohemagglutinin-P inoculation. The cellular component of the immune response was evaluated using whole-blood chemiluminescence and bacterial clearance assays. Results showed that infection with this AIV isolate suppressed T-cell function and enhanced macrophage phagocytic activity.