What is the relative contribution of genetic and various environmental factors to variation in the ability to mount an immune response? We measured antibody responsiveness to diphtheria-tetanus vaccine during the winter in free-ranging blue tits with a known nestling history to investigate (1) if nutritional status during the nestling stage has persistent effects on an individual's immune defence and (2) if immune responsiveness is heritable. There was no correlation between nutritional status during the nestling phase (measured as size-corrected body mass day 14 post-hatch) and antibody responsiveness as an adult. On the other hand, the heritability of responsiveness to diphtheria and tetanus, as estimated by parent-offspring regression, was 0.21+/-0.51 and 1.21+/-0.40 SE, respectively. Thus, while there was little evidence that natural variation in antibody responsiveness to these antigens reflected nutritional conditions during early life, responsiveness to at least one of the antigens (tetanus) had a strong genetic component.