Description: The aim of this study was to evaluate serum concentrations of serum amyloid A (SAA), haptoglobin (Hp), interferon-gamma (INF-gamma) and tumor necrosis factor-alpha (TNF-alpha) in dairy calves affected with diarrhoea during the first 4 weeks of life, with special reference to etiological agents. To study the acute phase reaction in infectious diarrhoea with E. coli k99, rota and corona viruses and Cryptosporidium parvum, 50 clinically healthy and 50 diarrhoeic calves 1Ц28 days of age were randomly selected. Blood samples were taken from jugular vein and serum concentrations of serum amyloid A (SAA), haptoglobin (Hp), interferon-gamma (INF-gamma) and tumor necrosis factor-alpha (TNF-alpha) were determined. Although all studied acute phase response indicators were significantly increased in diarrhoeic calves, SAA and Hp were two major acute phase proteins that were remarkably increased (p<0.05) compared to healthy animals. According to this study, the SAA and Hp correlated with faecal scores and proved to be reliable indicators of the severity of diarrhea (Spearman rho = 0.22 and 0.23, respectively, p=0.05)