Tuatara are the sole survivors of an entire order of reptiles that thrived during the age of the dinosaurs. Therefore, knowledge of their physiology is critical to understanding the phylogeny of reptiles. Previous studies of the immune system of the tuatara did not assess T cell function. We analyzed T cell function among six captive tuatara by assessing concanavalin A (Con A), phytohemagglutinin (PHA) and mixed lymphocyte reaction (MLR) induced T cell proliferation. Peripheral blood mononuclear cells from six out of six and four out of four tuatara tested exhibited significant proliferative responses to Con A and PHA, respectively, as measured by an MTT reduction assay. A lower level of proliferation was detected in an MLR. However, Con A activated lymphocytes were not cytotoxic for a xenogeneic murine mastocytoma cell line (P815).