A study was performed on cell suspension from peripheral blood and bone marrow aspirates and on cryostat sections from bone marrow biopsies in order to investigate the membrane phenotype of neoplastic B cells in chronic lymphocytic leukaemia (B-CLL). The immunological analyses, performed on 43 patients, included rosetting ability with sheep and mouse erythrocytes, evaluation of surface immunoglobulins and reactivity with anti-HLA-DR, UCHT 1 (OKT-3 like) and RFA-1 (OKT-1 like) monoclonal antibodies. The results demonstrate that neoplastic B lymphocytes in B-CLL display an identical phenotype in peripheral blood and bone marrow. Possible interpretations on the origin of proliferating cells in B-CLL are discussed.