Cattle chromosome 6 was scanned with 11 markers, ten microsatellites and thecasein haplotype, to identify quantitative trait loci (QTLs) affecting thefollowing milk production traits: milk yield, fat percentage, fat yield, proteinpercentage and protein yield. Twelve Finnish Ayrshire half-sib families with atotal of 480 sons were genotyped and used in a grand-daughter design. Intervalmapping was performed with a multiple-marker regression approach with a one-QTLand a two-QTL model, and the significance threshold values were determinedempirically using a permutation test. Across-family analysis with the one-QTLmodel revealed an effect on protein percentage (P < 0.05) and on milk yield (P <0.05). The analysis with the two-QTL model identified significant effects (P <0.05) on protein percentage, milk yield, and fat yield. Comparing these twocases, the results suggest the existence of two QTLs on chromosome 6 with aneffect on milk production traits. One of the QTLs was located around the caseingenes. As the other QTL was similar in location and effect to a QTL foundpreviously in Holstein-Friesians, an identity-by-descent approach could beapplied to fine map this region.