Elucidation of cellular processes and their changes at the level of protein expression and post-translational modification patterns may allow identification of novel proteins and thereby mechanisms involved in the pathogenesis of multigenic diseases. The aim of this study was to test cultured, nontransformed primary fibroblasts derived from human skin biopsies as a suitable model system for proteome analysis. Therefore soluble protein fractions were separated on several overlapping ultrazoom gels covering the pH range from 3.5-9. Correlation analysis of gel-pairs revealed a highly reproducible protein expression pattern within (intra-assay) and between (inter-assay) independent experiments of a single fibroblast cell line (intra-cell line comparison). Spot intensity variations were less than a factor of two for more than 80% of identical spots. In addition, inter-cell line comparison exhibits no significant variations in spot intensities. To achieve further improvements in reproducibility we generated master gels for each pH range by combining averaged spot information derived from two different cell lines each analysed by two independent experiments using the raw master gel algorithm of the Z3 image analysis software. The resulting reference images of primary human fibroblasts provided a basis for investigating regulation by extracellular stimuli and drugs as well as their alterations in patients with different diseases.