Seventy to 75 sons of each of six Holstein sires were assayed for genotypes at anumber of microsatellite loci spanning Chromosomes (Chrs) 1 and 6. The number ofinformative loci varied from three to eight on each chromosome in different sirefamilies. Linkage order and map distance for microsatellite loci were estimatedusing CRI-MAP. Estimates of QTL effect and location were made by using a leastsquares interval mapping approach based on daughter yield deviations of sons for305-d milk, fat, protein yield, and fat and protein percentage. Thresholds forstatistical significance of QTL effects were determined from interval mapping of10,000 random permutations of the data across the bull sire families and withineach sire family separately. Across-sire analyses indicated a significant QTLfor fat and protein yield, and fat percentage on Chr 1, and QTL effects on milkyield and protein percentage that might represent one or two QTL on Chr 6.Analyses within each sire family indicated significant QTL effects in five sirefamilies, with one sire possibly being heterozygous for two QTLs. Statisticallysignificant estimates of QTL effects on breeding value ranged from 340 to 640 kgof milk, from 15.6 to 28.4 kg of fat, and 14.4 to 17.6 kg of protein.