Blastogenic and cytotoxic responsiveness of peripheral blood leukocytes from the green turtle, Chelonia mydas were examined. Blastogenic responses were low level and showed considerable variation between animals. Blastogenesis in response to phytohemagglutinin and concanavalin A was observed through out all seasons. Responses to pokeweed mitogen and lipopolysaccharide were seasonal, appearing in spring. No blastogenic response to protein A or allogeneic leukocytes was observed. Cytotoxic responses to phytohemagglutinin and antibody-dependent cell-mediated cytotoxicity were of significant magnitude and, all animals responded in a similar manner. No spontaneous cytotoxic reactivity was observed.