Infection trials were performed to characterize experimental Eimeria zuernii coccidiosis parasitologically and clinically and to investigate the effects on weight gain and haemotologic parameters in affected calves. Three groups of calves were formed: Group 1 (n = 14) served as uninfected control group, group 2 (n = 11) was infected with 150,000 sporulated E. zuernii oocysts per calf, and group 3 (n = 16) was infected with 250,000 sporulated E. zuernii oocysts per calf. All infected animals shed oocysts and showed diarrhoea; a positive correlation could be shown between quantified oocyst excretion and faecal consistency. Measurements throughout the prepatent and the patent period revealed a marked influence of E. zuernii infection on weight gain, leukocyte concentration, haemoglobin, haematocrit, and mean cellular volume. Aberrations in these parameters were most pronounced in the highly infected group. The results of this study confirm that acute sublethal E. zuernii coccidiosis causes distinct loss of fluid and blood via intestine. This dominates also the haematological picture of the disease, which is mainly characterized by haemoconcentration. Leukocyte concentration was depressed during the early patent period, whereas it increased markedly from day 24 after infection on.