The purpose of the present study was to calculate the critical differences between two analytical results for 23 routinely used bovine clinical-chemical parameters. The critical difference can be used to judge whether the difference between two consecutive analytical results from the same animal is due to natural variation or not. From 20 cows, blood samples were collected once daily for five days, and the interindividual variance, the intra-individual variance, and the analytical variance were calculated using nested analysis of variance. The critical difference both in absolute values and in percentages was calculated from these variances. The main conclusions were that the critical difference in percentages was in general between 10 and 40 per cent but varied from 10.6 per cent for sodium to 280.2 per cent for lactate and that the analytical variation was generally in accordance with accepted standards, although the analytical variation for alkaline phosphatase, total serum protein, urea and fructosamine should be improved.