Sons (n = 71 to 75) of each of six Holstein sires were genotyped at 69microsatellite loci covering a total of 676 cM on chromosomes 3, 5, 9, 10, 13,15, 17, 20, 23, and 26. Estimates of quantitative trait loci (QTL) effect andlocation were made using a least squares interval mapping approach based ondaughter yield deviations of sons for 305 d milk, fat, and protein yield and fatand protein percentage. Thresholds for statistical significance of QTL effectswere determined from interval mapping of 10,000 random permutations of the dataacross the bull sire families and within each sire family separately. Analysescombining data across sires indicated the presence of QTL affecting milk, fat,and protein yield on chromosomes 20 and 26 and a QTL affecting fat and proteinpercentage on chromosome 3. Analyses within each sire family separatelyindicated the presence of segregating QTL in at least one family on 7 of the 10chromosomes. Statistically significant estimates of QTL effects on breedingvalue ranged from 438 to 658 kg of milk, from 17.4 to 24.9 kg of fat, 13.0 to17.0 kg of protein, 0.04 to 0.17% fat, and 0.07 to 0.10% protein.