Stresses are environmental factors which restrict growth or cause a potentially adverse change in an organism. The exposure of developing organisms to environmental stresses may have several physiological consequences including a decrease in immunocompetence. However, mounting an immune response against a foreign antigen may in itself constitute a cost for developing organisms. This cost has potentially long-term consequences for adult function and fitness. This study examines the growth and developmental stability of Japanese quail++ chicks challenged by three non-pathogenic antigens: sheep red blood cells, which assess T-cell-dependent immune responses, and Mycoplasma synoviae and Newcastle disease virus, which assess T-cell-independent responses. Increases in both body mass and wing length were significantly reduced in antigen-challenged birds compared to control birds. Fluctuating asymmetry (FA) in the masses of primary feathers increased from the innermost (1) to the outermost (10) position on the wing. In addition, antigen challenge by M. synoviae and sheep red blood cells was associated with an increase in FA. The cell-mediated response measured by reaction to phytohaemagglutinin was significantly depressed in M. synoviae-challenged birds. White blood cell counts, except for monocytes, were elevated in response to all three antigen treatments. Total plasma protein and haematocrit also differed between treatments but exhibited no clear relationship to antigen challenge. Immune responses clearly impose a stress on developing chicks. Additional research will be required to determine the long-term consequences of developmental stress and assess the selective forces that influence the strength of the immune responses of chicks.