Although bronchoalveolar lavage has been used as a research and clinical tools for more than two decades to investigate the cellular and soluble components of the lower respiratory tract, its exact protein composition has never been established. In this context, proteins of human bronchoalveolar lavage fluids (BALF), obtained by washing the epithelial lining fluid of the lungs with phosphate-buffered saline, were analyzed by two-dimensional electrophoresis (2-DE) under denaturing and reducing conditions. To characterize the widest amount of proteins, an analytical map of human bronchoalveolar lavage fluid proteins has been created from a pool of BALF from various patients. The resulting map comprises 211 silver-stained spots in the range of pI 3.5-10 and molecular mass 5-100 kDa. We identified 182 spots by microsequence analysis and by matching with human blood plasma and the Macrophage Like Cell line reference 2-DE maps available from the SWISS-2DPAGE database. The human bronchoalveolar lavage fluid was found to contain 61 different proteins or isoforms thereof. Most of the proteins had low molecular masses (< 35 kDa) and rather acidic isoelectric points (pI; 4 < pI < 7). The proteins in the lavage either are produced locally or originate from plasma. Two unknown proteins were identified and are currently under investigation.