ZAP-70 tyrosine kinase is involved in signalling pathways following T-cell receptor stimulation and was originally described only in T cells and natural killer cells. ZAP-70 expression has been reported in normal mouse B lineage cells and in human malignant B lymphocytes, mainly in chronic lymphocytic leukemia (CLL) where it correlates with clinical outcome. We analyzed several B-cell lines and ex vivo malignant B cells, ranging from acute lymphoblastic leukemia to multiple myeloma and reflecting different stages of B-cell differentiation, and they showed ZAP-70 expression regardless their maturation stage. We then analyzed by Western blot and flow cytometry different human normal B-lymphocyte subpopulations: naïve, germinal center and memory B cells from tonsils, CD19+ CD5+ cells from cord blood and CD19+ lymphocytes from peripheral blood. All expressed ZAP-70 protein, though at different levels depending on their differentiation, activation and tissue localization. In addition, ZAP-70 expression levels could be modulated following stimulation via the B-cell receptor. These findings implicate a potential role of ZAP-70 in the signalling pathway of B lymphocytes at different maturational stages, indicate that ZAP-70 expression is not a CLL-specific feature among B-cell malignancies and suggest that the absence of ZAP-70 rather than its presence should be considered abnormal for malignant B lymphocytes.