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Innate immunity provides a fast-acting and nonspecific defense against microbial infection, and appears to have particular importance in the immune response of ectothermic vertebrates. Chelonians are a globally distributed and diverse group, yet little is known about their basic immune function. The chelonian family Chelydridae is made up of two genera (Chelydra and Macrochelys), represented in our study by the widespread common snapping turtle (Chelydra serpentina; CST) and the southeast USA endemic alligator snapping turtle (Macrochelys temminckii; AST). Our goal was to quantify the innate immune response of the family Chelydridae, using the antibacterial activity of plasma as a measure of immune function. Our results show that the plasma of both species has strong antibacterial properties, but CST plasma kills a higher percentage of bacteria than AST plasma. In addition, while both species showed the highest antibacterial activity at 25 to 30 degrees C, CST plasma retained its antibacterial properties at lower and higher temperatures than AST plasma. Our results indicate that, like many ectotherms, Chelydridae have a relatively strong innate immune response. The stronger, more robust immune response of CSTs compared with ASTs is likely correlated to the differences in geographic ranges but may also have implications for each species' tolerance to anthropogenic habitat degradation and global climate change.