textlessptextgreaterCoccidiosis, a parasitic disease of the intestinal tract caused by members of the genera and , is one of the most common and costly diseases in chicken. The aims of this study were to assess the effect of the challenge and level of variability of measured parameters in chickens during the challenge with . Furthermore, this study aimed to investigate which parameters are the most relevant indicators of the health status. Finally, the study also aimed to estimate accuracy of prediction for traits that cannot be measured on large scale (such as intestinal lesion score and fecal oocyst count) using parameters that can easily be measured on all animals. The study was performed in 2 parts: a pilot challenge on 240 animals followed by a large-scale challenge on 2,024 animals. In both experiments, animals were challenged with 50,000 oocysts at 16 d of age. In the pilot challenge, all animals were measured for BW gain, plasma coloration, hematocrit, and rectal temperature and, in addition, a subset of 48 animals was measured for oocyst count and the intestinal lesion score. All animals from the second challenge were measured for BW gain, plasma coloration, and hematocrit whereas a subset of 184 animals was measured for intestinal lesion score, fecal oocyst count, blood parameters, and plasma protein content and composition. Most of the parameters measured were significantly affected by the challenge. Lesion scores for duodenum and jejunum ( textless 0.001), oocyst count ( textless 0.05), plasma coloration for the optical density values between 450 and 490 nm ( textless 0.001), albumin ( textless 0.001), α1-globulin ( textless 0.01), α2-globulin ( textless 0.001), α3-globulin ( textless 0.01), and β2-globulin ( textless 0.001) were the most strongly affected parameters and expressed the greatest levels of variation. Plasma protein profiles proved to be a new, reliable parameter for measuring response to . Prediction of intestinal lesion score and fecal oocyst count using the other parameters measured was not very precise ( textless 0.7). The study was successfully performed in real raising conditions on a large scale. Finally, we observed a high variability in response to the challenge, suggesting that broilers response to has a strong genetic determinism, which may be improved by genetic selection.textless/ptextgreater