The main objective of this study was to estimate the proportion of total geneticvariance attributed to a quantitative trait locus (QTL) on Bos taurus autosome 6(BTA6) for milk production traits in the German Holstein dairy cattlepopulation. The analyzed chromosomal region on BTA6 spanned approximately 70 cM,and contained 6 microsatellite markers. Milk production data were obtained fromroutine genetic evaluation for 4500 genotyped German Holstein bulls. Technicalaspects related to the estimation of model parameters for a large data set fromroutine genotype recording were outlined. A fixed QTL model and a random QTLmodel were introduced to incorporate marker information into parameterestimation and genetic evaluation. Estimated QTL variances, expressed as theratio of QTL to polygenic variances, were 0.04, 0.03, and 0.07 for milk yield;0.06, 0.08, and 0.14 for fat yield; and 0.04, 0.04, and 0.11 for protein yield,in the first 3 parities, respectively. The estimated QTL positions, expressed asdistances from the leftmost marker DIK82, were 18, 31, and 17 cM for milk yield;25, 17, and 9 cM for fat yield; and 16, 30, and 17 cM for protein yield in the 3respective parities. Because the data for the parameter estimation wellrepresented the current population of active German Holstein bulls, the QTLparameter estimates have been used in routine marker-assisted genetic evaluationfor German Holsteins.