We describe optimization of a peripheral blood mononuclear leukocyte proliferation assay and development of an interleukin-2 receptor (IL-2R) expression assay for bottlenose dolphins (Tursiops truncatus). Peripheral blood mononuclear leukocytes obtained from both Sea World (February 1993) and the Naval Command Control and Ocean Surveillance Center (March 1993) (San Diego, California, USA) were stimulated with the mitogens concanavalin A (ConA) and phytohemagglutinin (PHA) and evaluated for optimum proliferation and IL-2R expression. Based on these optimization assays, standard conditions were established and used to assess immune function in a population of apparently healthy, free-ranging bottlenose dolphins from Sarasota Bay, Florida (USA) in June 1993. A positive correlation was observed between proliferation assays using ConA and PHA as the stimulants. However, IL-2R expression induced by both mitogens differed significantly.