An increase in concern for captive and free-ranging marine mammal health has resulted in the development of techniques to assess their immune status. Peripheral blood is often the only available window into the immune system of these animals. Leukocyte subpopulations in the peripheral blood of many species fluctuate in response to various conditions, and indeed, shifts in leukocyte percentages can provide clues as to an individual's immune status. To this end, seven leukocyte-specific monoclonal antibodies (mAbs) raised in this laboratory against bottlenose dolphin (Tursiops truncates) -- and cross reactive with all members of the family Delphinidae -- are currently being screened to identify those that recognize proteins expressed differentially in sick and healthy killer whales (Orcinus orca). In addition, the value of putative T and B lymphocyte-specific mAbs as indicators of aberrant health is being investigated. Clinical evaluations that include gross veterinary examinations, microbiological, hematological, and behavioral analyses of captive killer whales are being compared to leukocyte subpopulation phenotypes of peripheral blood as determined by flow cytometry subsequent to staining with selected mAbs. Preliminary data demonstrate that a correlation exists between certain immunophenotypic profiles and clinical health. These initial results indicate that some of these mAbs may be valuable prognostic and diagnostic tools and serve as useful predictors of potential health risk or as indicators of immediate acute or chronic infection.