The energy cost of adaptive immune activation in endotherms is typically quantified from changes in resting metabolic rate following exposure to a novel antigen. An implicit assumption of this technique is that all variation in energy costs following antigenic challenge is due solely to adaptive immunity, while ignoring potential changes in the energy demands of ongoing bodily functions. We critically assess this assumption by measuring both basal metabolic rate (BMR) and exercise-induced maximal metabolic rate (MMR) in house sparrows before and after the primary and two subsequent vaccinations with either saline (sham) or two novel antigens (keyhole limpet haemocyanin and sheep red blood cells; KLH and SRBC, respectively). We also examined the effect of inducing male breeding levels of testosterone (T) on immune responses and their metabolic costs in both males and females. Although there was a moderate decrease in KLH antibody formation in T-treated birds, there was no effect of T on