It has been suggested that alterations in bronchoalveolar lavage fluid (BALF) reflect pathologic changes in the lung. Cytoplasmatic enzymes such as lactate dehydrogenase (LDH), alkaline phosphatase (ALP), and LDH isoenzymes are recognized indicators of cell damage or death. The aim of this study was to determine whether there is a relation between the enzyme activity and the cell types present in BALF. Therefore, BALF samples obtained from patients with various pulmonary disorders were studied. Out of these samples a group with mainly polymorphonuclear neutrophils (PMNs; n = 15; Group I) and another with mainly alveolar macrophages (AMs; n = 10; Group II) were selected. Additionally, the value of analysis of lysed cells in BALF for assessment of LDH-isoenzyme patterns was examined. The cell-free fraction of BALF of Group II showed lower LDH and ALP activity compared to Group I. The LDH-isoenzyme pattern also differed, with the LDH3/LDH5 ratios being lower in all BALF samples with predominantly PMNs than in BALF samples with predominantly AMs. Lysis of the cells present in the BALF samples by sonication prior to LDH-isoenzyme analysis provided no additional information beyond that found by analysis of the cell-free BALF. In conclusion, determination of enzyme activity appears to be useful in monitoring pulmonary inflammation.