The aim of this study was to investigate whether the demonstration of plasma cells (PC), which are normally absent in bronchoalveolar lavage (BAL) fluid, facilitates differentiation among pulmonary disorders. Initial BAL fluid samples of 1,260 patients were analyzed. In 83 of these, PC were found. Of these 83, 47 were obtained from individuals suffering from extrinsic allergic alveolitis (EAA). The number of PC in BAL fluid from EAA patients was found related to the time between antigen exposure and BAL. Drug-induced pneumonitis appeared to be another disorder with a high percentage of cases with PC in the BAL fluid (35.7 percent). Therefore, we conclude that determination of PC in BAL fluid has differential diagnostic value in discriminating among interstitial lung diseases of various origins. However, the exact role of BAL fluid and PC and the link to clinical manifestations of these diseases needs further investigation.