A trade–off between immunity and growth has repeatedly been suggested, mainly based on laboratory and poultry science, but also from experiments where parasitism intensity was manipulated in field bird populations. However, as resource allocation to different activities (or organs) during growth is difficult to manipulate, this trade–off has only been experimentally tested by studying the effects of non–pathogenic antigens. By providing some nestling magpies (Pica pica) with methionine, a sulphur amino acid that specifically enhances T–cell immune response in chickens, we investigated this trade–off by directly affecting allocation of limited resources during growth. Results were in accordance with the hypothetical trade–off because nestlings fed with methionine showed a lower growth rate during the four days of methionine administration, but a larger response when fledglings were challenged with phytohaemagglutinin (a measure of the intensity of T–lymphocyte–mediated immune responsiveness) than control nestlings. Surprisingly, we found that control and experimental nestlings fledged with similar body mass, size and condition, but experimental nestlings suffered less from blood parasites (Haemoproteus) and had fewer lymphocytes (a widely used measure of health status) than control nestlings, suggesting a negative effect of blood parasites or other pathogens on nestling growth.