1. Energy saving is often suggested as the basis of a resource trade-off between immunocompetence and other fitness-relevant traits. This suggests that the energetic cost of an immune response is significant and sufficient to force trade-offs. To date, few studies have investigated the energetic cost of the humoral component of the immune system in birds and furthermore, existing results are contradictory.2. We addressed this question through two experiments. In experiment 1, the basal metabolic rate (BMR) of Collared Doves,Streptopelia decaocto, challenged with sheep red blood cells (SRBC) was compared with the BMR of control birds. The energetic cost of immunity on host life-history strategies was compared with another physiological activity, thermoregulation, in experiment 2 to assess its significance.3. Experiment 1 showed that antibody production against SRBC increased BMR of birds, with a peak of energy expenditure 7 days after immune activation (+8·5% BMR). In addition, we found that among birds fedad libitum, those mounting a stronger immune response lost significantly more mass than controls or birds mounting a low immune response. In experiment 2, we found the cost of thermoregulation to be 5·27% BMR °C−1.4. If results from experiment 1 primarily suggested that an energy-based trade-off was expected between immune functions and other fitness-related traits, experiment 2 showed that the magnitude of this energetic cost corresponded to that used during low levels of thermoregulation. Consequently, we suggest that energy saving is not the central mechanism of a trade-off mediating immunocompetence. We provide some evidence that the degradation of body condition should be considered as an important additional cost of humoral immunity in the context of a resource-based trade-off.Functional Ecology(2005)19, 110 –118 [ABSTRACT FROM AUTHOR]