Inflammation and activation of the acute phase response (APR) are energetically demanding processes that protect against pathogens. Phytohaemagglutinin (PHA) and lipopolysaccharide (LPS) are antigens commonly used to stimulate inflammation and the APR, respectively. We tested the hypothesis that the APR after an LPS challenge was energetically more costly than the inflammatory response after a PHA challenge in the fish-eating Myotis bat (Myotis vivesi). We measured resting metabolic rate (RMR) after bats were administered PHA and LPS. We also measured skin temperature (Tskin) after the LPS challenge and skin swelling after the PHA challenge. Injection of PHA elicited swelling that lasted for several days but changes in RMR and body mass were not significant. LPS injection produced a significant increase in Tskin and in RMR, and significant body mass loss. RMR after LPS injection increased by 140-185% and the total cost of the response was 6.50 kJ. Inflammation was an energetically low-cost process but the APR entailed a significant energetic investment. Examination of APR in other bats suggests that the way in which bats deal with infections might not be uniform.