The aim of this study was to evaluate the clinical and biochemical importance of sialic acid (SA) serum concentrations (total, lipid-bound and protein-bound) in calves suffering from infectious bovine keratoconjunctivitis (IBK). The study included 18 calves with IBK (Group 1) and I I healthy calves (Group 11). Blood samples were collected from all calves for the determination of serum SA and glucose concentrations. Eye lesions were scored as mild, moderate and severe based on severity of the clinical symptoms. Mean total sialic acid (TSA), lipid-bound sialic acid (LBSA) and protein-bound sialic acid (PBSA) concentrations of the calves with IBK were 106,9 +/- 3,6 mg/dl, 54,81 +/- 2,78 mg/dl, 52,1 +/- 3,1 mg/dl, respectively. Values of TSA, LBSA and PBSA determined for healthy calves were 76,4 +/- 3,1 mg/dl, 38,4 +/- 2,1 mg/dl and 38 +/- 3,7 mg/dl, respectively. Serum TSA and LBSA values measured in animals from group I were significantly higher than those of animals from group 11 (p<0.001). As the severity of eye lesions increased, the LBSA serum concentration was also increased reaching 47.5 +/- 1.8 mg/dl; 50.8 +/- 0.9 mg/dl; 66.1 +/- 6.1 mg/dl for calves presenting mild, moderate and severe lesions, respectively.