In a study to initially explain the prevalence of nematodiasis (Pharurus pallasii) in D. leucas from the Churchill River Basin, blood samples were taken to assess immunologic responsiveness. Analyses of B- and T-lymphocytes were performed on blood from 7 Churchill whales that bad been in captivity from 3 months to 12 years (New York and Mystic Aquaria). The antisera used to identify the T-lymphocyte subpopulations was the OKT series of mouse monoclonal antibodies (ortho Pharmaceutical Corporation). Cells with antigens reacting with antibodies B₁, OKT3, OKT4, OKT8 and OKT11, were counted and separated with a fluorescence activated cell sorter and fractions observed by light and electron microscopy. Whales recently brought into captivity had four-fold ratios of "T-suppressor" to "T-helper." Churchill whales that had been in captivity for 10 years had equal "T-suppressor" and "T-helper." In two recent captives, which became ill, the level of 'helper" T-cells exceeded the level of "T-suppressor" cells. Histology on one of these whales revealed evidence of B-cell stimulation. This data indicates that monoclonal antibodies raised to human cell surface antigens cross-reacts in whales and provides a means of assessing lymphoid stimulation.