We assessed whether levels of heavy metals in blood have detrimental effects on the health of prefledgling black kites (Milvus migrans) exposed to emissions from a solid waste incinerator near Madrid, central Spain. As health indexes, we considered body mass corrected by structural size as a general indicator of nutritional condition and white blood cell counts (especially the heterophil:lymphocyte ratio) as an indirect measure of immune function. Nutritional condition was positively related to levels of Zn, an essential element with a major role in many physiological pathways, while the opposite trend was found for the essential Cu, the concentration of which tends to increase with decreasing nutritional condition. We found an interaction between Zn and Cd, indicating that high levels of Zn produce smaller effects of Cd on body mass. Potential immunological stress, expressed as an increase in the heterophil:lymphocyte ratio, caused by the toxic Cd increased as levels of Cu decreased. Thus, nestlings with low levels of Cu and poor nutritional condition suffered more immunological stress due to Cd contamination than nestlings with higher levels of Cu. We also found a negative interaction between Zn and Cd, indicating a response of essential Zn to increasing levels of Cd, which may be indicative of the negative effects of Cd on the immune system. These results suggest that the participation of metallothioneins in detoxification and metal regulation may also indirectly enhance the adrenal stress response to contaminants, probably through a competition of Cd and Zn for Zn binding sites on metallothioneins, reducing the ability of Cd to stress the immune system.