Ruminal fluid and blood samples were analysed in 39 calves with poor suckling behaviour and reduced milk intake (group 1) and in 21 calves with normal suckling behaviour (group 2). The pH level and chloride concentration was determined in the ruminal fluid as well as in the blood. In addition Gram stains and subsequent microscopical examinations of the ruminal fluids were carried out. Blood samples were submitted for blood gas analysis. Notable differences were found between the two groups. The pH level of the ruminal fluid was less than 5.5 in 29 of the 39 calves in group 1 (74.4%); in contrast, the pH was always above 5.5 in group 2. In group 1, 22 calves with a pH of less than 5.5 also revealed a metabolic acidosis of the blood. Mainly gram-positive bacteria were found in the ruminal fluid of 74.4% of the calves in group 1. On the other hand, 76.2% of the calves in group 2 revealed a mainly gram-negative flora. There was no difference in the chloride concentration, either in the blood or in the ruminal fluid and the values showed a high range of scatter.