Patients with B chronic lymphocytic leukemia (B-CLL) demonstrate a high variability in disease activity, from a benign monoclonal lymphocytosis to a rapidly fatal condition. Progressive B-CLL is related to a high expression of different growth factor receptors on the leukemic cells and to a high proliferative in vitro response to Staphylococcus aureus strain Cowan 1 (SAC). As the expression of membrane IL-1 alpha (mIL-1 alpha) indicates B lymphocyte activation, we have investigated mIL-1 alpha on cells from patients at different stages of the disease. Total cellular levels of IL-1 alpha were measured by flow cytometry of permeabilized cells and compared with CD5, CD19, CD25 and IgM expression on the cell surface. mIL-1 alpha is upregulated, both in leukemic and normal lymphocytes, in response to sIgM cross-linking with SAC or phorbol ester activation. A significantly higher expression of mIL-1 alpha was found in cells from patients with a clinically benign form of the disease.