We attempted to verify the clonality of proliferating cells in broncho-alveolar lavage fluid (BALF) by using a sophisticated technique known as kappa-lambda imaging. We applied this method to BALF lymphocytes from 3 cases suspected of having primary pulmonary B-cell lymphoma, by transbronchial lung biopsy, and from 2 cases with B-cell lymphoma revealing abnormal shadows. (One was diagnosed as small cell carcinoma, the other mycotic infection). B-cell monoclonality was found in the former 3 cases, but not in the latter 2 cases. We conclude that kappa-lambda imaging in BALF lymphocytes is useful for diagnosing B-cell lymphoma of the lung.