Clinical use of the antitumor antibiotic bleomycin (BLM) is associated with the development of interstitial pulmonary fibrosis. Shortly after acute lung damage caused by intratracheal (it) administration of BLM to experimental animals there is an influx of inflammatory cells which are believed to modulate the process of fibrosis. This study was undertaken to determine that subpopulations of lymphocytes were in the bronchoalveolar lavage (BAL) cell population of C57BL/6J mice at various times after a single it dose of BLM and to determine whether BAL T-lymphocytes were activated after BLM treatment. The BAL lymphocyte population was analyzed by differential cell analysis and flow cytometry utilizing monoclonal antibodies specific for lymphocyte subpopulations. The majority of lymphocytes in the BAL of control and BLM-treated mice were T-lymphocytes, with less than 10% being B-cells. During the first 7 days after BLM the number of Lyt-2⁺ T-cells exceeded L3T4⁺ T-cells while in control mice the reverse was observed. The percentage of BAL lymphocytes expressing the IL-2 receptor did not change significantly at 3 and 7 days after BLM treatment, but was significantly decreased at Day 14. In contrast, the total number of lymphocytes expressing the IL-2 receptor was increased at all time points investigated. These results demonstrated that the majority of lymphocytes in the BAL were T-cells and that while the percentage of activated lymphocytes did not increase following BLM treatment, the absolute number did and this increased number of activated lymphocytes may be important in the disease process.