1. Recently, there has been much interest in physiological trade-offs between parasite resistance and fitness-related traits such as secondary sexual characters or reproductive effort. More specifically it has been suggested that (i) energetically costly activities may suppress the immune system and (ii) that this immunosuppression is caused by costly immune defences competing with other bodily demands for scarce resources, e.g. energy. 2. The possibility was investigated of an energetically based trade-off between humoral (antibody-based) immunocompetence and other costly activities, by immunizing Blue Tits, Parus caeruleus, with novel antigens (proteins) thereby inducing antibody responses, and performing two experiments. In experiment i, one group of birds was subjected to cold stress, thereby increasing their daily energy expenditure and the effect on immune responsiveness was investigated. In experiment 2, the basal metabolic rate (BMR) of immunized birds was measured to investigate the energetic costs of mounting the antibody responses. 3. In experiment I, birds subject to increased energy turnover had significantly lower antibody responses, consistent with the hypothesis that environmental stress could suppress immunocompetence. However, in experiment 2 the energetic costs of these antibody responses were found to be low and at most 8-13% of BMR, indicating that adaptive resource allocation of energy was an unlikely explanation for the lowered immune responsiveness in the cold stress treatment (experiment 1). 4. It is concluded that our data provide some support to the idea that there may be a trade-off between immunocompetence and energetically costly activities such as thermoregulation, reproduction or mate attraction, although this trade-off may not necessarily be based on energy or nutrient limitation (i.e. resource allocation models). Two non-energetic explanations are briefly discussed, one adaptive and one non-adaptive, that could explain the immunosuppression in our study as well as in other behavioural and ecological contexts.