Fifty-nine patients with a leukaemic B-lymphocytic malignancy ("CLL") were studied. According to the Kiel classification, 29 patients had chronic lymphocytic leukaemia (CLL) and 30 had immunocytoma (IC). Cell surface immunoglobulin staining showed mu heavy chain phenotype in 14 patients, mu in 35, gamma in 7; in cells from 3 patients the staining was too weak to permit identification. The light chain phenotype was kappa in 39 patients, lambda in 17, and unidentified in 3. The immunoglobulin isotypes differed between the diagnoses. The gamma chain phenotype was found only in IC patients (p less than 0.02), and more CLL than IC patients showed a lambda chain phenotype (p less than 0.04). Blood lymphocytes from IC patients contained more T cells than CLL cell samples (p less than 0.002). No prognostic difference was found between the CLL and IC group. Compared to the lambda phenotype, the kappa phenotype was associated with a poorer prognosis in the IC group, but with a better prognosis in the CLL group. IC patients with mu phenotype had a poorer prognosis than those with gamma phenotype. Low relative T cell numbers were associated with a poor survival (p less than 0.01).