Studies of lymphocyte markers in a patient with Sjögren's syndrome who exhibited histologically benign lymphoproliferation in the lung revealed a malignant cell clone. T and B cells were quantitated according to their ability to form spontaneous rosettes with sheep erythrocytes and to fluoresce with fluorescein-conjugated antiserums, respectively. Circulating lymphocytes were 66 percent T cells (N = 58 +/- 2 per cent) and 14 percent B cells (N = 22+/- 1 percent), the latter exhibiting normal polyclonal distribution of membrane immunoglobulins. However, lymphocyte suspensions obtained from fresh lymph node and from biopsy specimens from a lymphoid lung nodule revealed 95 percent and 88 percent B cells, with 1 percent and 2 percent T cells, respectively. Moreover, when cryostat-frozen sections from both tissues were reacted with each of the heavy and light chain-specific antiserums, most cells demonstrated the presence of intracytoplasmic mu kappa immunoglobulin exclusively. Twenty-two months later, a clinically and histologically classic lymphoma developed. Repeat marker studies performed on cells freshly isolated and on frozen sections from the histologically malignant lymph node revealed persistence of the monoclonal marker on most cells.