We studied the effects of a concurrent challenge on slow-growing broilers with 1) airborne particles of 2 sizes: fine dust (smaller than 2.5 microns) and coarse dust (between 2.5 and 10 microns) that were directly collected from a broiler house and 2) lipopolysaccharide on intratracheal immunizations with the specific antigen human serum albumin (HuSA) and measured primary and secondary systemic (total) antibody responses and (isotype-specific) IgM, IgG, and IgA responses at 3 and 7 wk of age. All treatments affected immune responses at several ages, heart morphology, and BW gain, albeit the latter only temporarily. Dust particles significantly decreased primary antibody (IgT and IgG) responses to HuSA at 3 wk of age but enhanced IgM responses to HuSA at 7 wk of age. Dust particles decreased secondary antibody responses to HuSA, albeit not significantly. All of the birds that were challenged with dust particles showed decreased BW gain after the primary but not after the secondary challenge. Relative heart weight was significantly decreased in birds challenged with coarse dust, fine dust, lipopolysaccharide, and HuSA at 3 wk of age, but not in birds challenged at 7 wk of age. Morphology (weight, width, and length) of hearts were also affected by the dust challenge at 3 wk of age. The present results indicate that airborne dust particles obtained from a broiler house when intratracheally administered at an early age affect specific humoral immune responsiveness and BW gain of broilers to simultaneously administered antigens differently than when administered at a later age. The hygienic status of broiler houses at a young age may be of importance for growth and immune responsiveness, and consequently, for vaccine efficacy and disease resistance in broilers. The consequences of our findings are discussed.