We studied the malignant lymphomas that developed in patients with Sjögren's syndrome and the antecedent benign salivary-gland lesions to determine their cellular characteristics. We used an immunoperoxidase technic that identified intracellular gamma, alpha and mu heavy chains and kappa and lambda light chains. In six of nine patients, the lymphomas were composed of cells containing intracytoplasmic immunoglobulin that was exclusively IgMK. The benign lymphoepithelial salivary-gland lesions preceding these malignant tumors consisted of approximately equal numbers of lymphoid cells containing either kappa or lambda light chains. Thus, in some patients with Sjögren's syndrome, there may be a progression in the lympho-proliferative lesions from a polyclonal infiltrate to a monoclonal neoplasm. Intracytoplasmic immunoglobulin identifies six of the nine cases as being B-cell in origin.