As part of a whole genome scan undertaken to detect quantitative trait loci(QTL) affecting milk yield and composition, we have genotyped a granddaughterdesign comprising 1152 sons for six microsatellite markers spanning bovinechromosome 20. An analysis performed across families provided strong evidence(experiment-wise P-values < 0.01) for the presence of a QTL affecting primarilyprotein percentage towards the telomeric end of the chromosome. A founder sire,shown in a previous study to segregate for a similar QTL in the correspondingchromosome region, was characterized by 29 and 57 sons and maternal grandsons,respectively, in the present design. Sorting corresponding sons and grandsons bypaternal or grandpaternal allele provided significant evidence for thesegregation of a QTL on chromosome 20. Altogether these results confirm thelocation of a QTL affecting milk production on bovine chromosome 20.