mapping economic trait loci for somatic cell score in holstein cattle using microsatellite markers and selective genotyping markerassisted selection mas uses genetic marker genotypes to predict an animals production potential and will provide additional selection information for progeny testing with the discovery of highly polymorphic microsatellite markers the tools now exist to begin the search for economic trait loci etl which is the first step toward mas the objective of this study was to identify etl for somatic cell score in an existing holstein population using the granddaughter design sons from seven grandsire families were genotyped with 20 autosomal microsatellites from five chromosomes 4 8 13 17 23 with an emphasis on chromosome 23 which is the location of the bovine major histocompatibility complex bola selective genotyping was used to reduce the number of genotypes required in which the 10 highest and 10 lowest sons from the phenotypic distribution curve were tested 140 sons in seven families one marker 513 located near bola showed evidence of an etl in three of five polymorphic families additional sons were genotyped from the five families to estimate the effect and to compare selective and complete genotyping both methods detected an etl at marker 513 but in different families this study provides evidence of the usefulness of microsatellite markers and the granddaughter design in the detection of etl however additional markers need to be evaluated to determine the usefulness of selective genotyping based on the results from the 20 studied markers the most likely position of a somatic cell score etl lies near marker 513 located on chromosome 23