The potential for Fusarium mycotoxins to modulate immunity was studied in chickens raised to 10 weeks of age using an enteric coccidial infection model. Experimental diets included: control, diets containing grains naturally contaminated with Fusarium mycotoxins, and diets containing contaminated grains + 0.2% polymeric glucomannan mycotoxin adsorbent (GMA). Contaminated diets contained up to 3.8 microg/g deoxynivalenol (DON), 0.3 microg/g 15-acetyl DON and 0.2 microg/g zearalenone. An optimized mixture (inducing lesions without mortality) of Eimeria acervulina, E. maxima and E. tenella was used to challenge birds at 8 weeks of age. Immune parameters were studied prior to challenge, at the end of the challenge period (7 days post-inoculation, PI), and at the end of the recovery period (14 days PI). Total serum immunoglobulin (Ig) A and IgG concentrations in challenged birds fed the contaminated diet were higher than controls at the end of the challenge period. Serum concentration of IgA, but not IgG, was significantly decreased at the end of the recovery period in birds fed the contaminated diet. The percentage of CD4+ and CD8+ cell populations in blood mononuclear cells decreased significantly at the end of the challenge period in birds fed the control or the contaminated diet compared to their percentages prior to challenge. The pre-challenge percentage of CD8+ population was restored at the end of the recovery period only in birds fed the control diet. Interferon-gamma (IFN-gamma) gene expression in caecal tonsils was up-regulated in challenged birds fed the contaminated diet at the end of the challenge period. No significant effect of diet was observed on oocyst counts despite the changes in the studied immune parameters. It was concluded that Fusarium mycotoxins modulate the avian immune system. This modulation involves alteration of gene expression but apparently does not enhance susceptibility or resistance to a primary coccidial challenge.