A granddaughter design containing five half-sib families from German Holstein-Friesian cattle was subjected to QTL analysis starting from the hypothesis of the existence of more than one QTL on chromosome BTA 6 affecting milk yield, fat yield, protein yield and content of fat and protein. The marker map consisted of 16 microsatellite markers with marker heterozygosity varying from 0.44 to 0.94. Two statistical methods were used: least squares (LS) and residual maximum likelihood (REML) allowing for two QTL simultaneously. The test statistics were calculated in steps of one cM along the chromosome. Significant QTL at the chromosome-wise 5% level according to the permutation test critical value were detected mainly in single families. The results were in conformance with the findings of several previous studies with approximate positions of putative QTL at 49 cM for milk yield, at 70 cM for fat and protein yield, and at 46 cM for protein content. Further QTL positions were suggested mostly for yield traits and protein content in the area of the casein gene cluster at 90...95 cM. The results of the two-QTL model analyses when using LS led to family specific inferences of a second QTL for fat yield and content of protein and fat, partly supported by the epistasis model.