Colonization of human skin with Staphylococcus aureus is a common feature in a variety of dermatologic diseases. In order to reproducibly investigate the adherence of Staphylococcus aureus to human epidermal cells, an in vitro assay was established using the biotin/streptavidine labeling system and the HaCaT cell line. This assay was used to define the role of several Staphylococcus aureus surface proteins with regard to their function in the staphylococcal adhesion process. Our studies included the standard laboratory strain Newman as well as its genetically constructed mutants DU5873, DU5852, DU5854, and DU5886 generated by allele replacement or transposon mutagenesis, which are deficient in the elaboration of staphylococcal protein A (spa), clumping factor (clfA), coagulase (coa), and the fibronectin-binding proteins A and B (fnbA/B), respectively. In comparison with strain Newman all mutants showed remarkably reduced adherence to the HaCaT keratinocyte cell line in our assay, yielding only between 43% and 60% of the adherence capacity of strain Newman after 60 min. Bacterial adherence could be re-established by introducing the cloned wild-type genes for the surface proteins on shuttle plasmids into the chromosomally defective mutants, thus suggesting a pathogenetic role of these proteins in the attachment of Staphylococcus aureus to human keratinocytes. Bacterial adherence was additionally enhanced by alkaline pH-values that are characteristic for skin conditions with epidermal barrier dysfunction. The use of Staphylococcus aureus mutant strains, deficient in the elaboration of defined proteins, allows specific investigation of colonization and virulence factors of this dermatologic relevant microorganism.