In order to compare flow cytometry with the conventional peroxidase anti-peroxidase method for the immunophenotyping of bronchoalveolar lavage fluid (BALF) lymphocytes, we studied BALF samples from 27 patients with various interstitial lung diseases. The results achieved with both methods were consistent concerning CD3+ pan T cells, CD4+ T helper/inducer, CD8+ T suppressor/cytotoxic and CD57+ natural killer cells. In contrast, a statistically significant lower anti-HLA-DR positive subset was obtained with flow cytometry than with the immunoperoxidase method (p less than 0.005). Since regression analyses and reliability counts showed further agreement between the methods, we conclude that flow cytometric immunophenotyping of BALF lymphocytes leads to similar, if not better, subset analyses than the immunoperoxidase method.