The aim of this study was to establish how effective is the use of immunohistochemistry on formalin-fixed bone marrow in diagnosing low-grade B-cell neoplasms. We investigated a series of 41 consecutive patients with bone marrow involvement for whom no other diagnostic tissues were available. The sections were stained with the following antibodies: CD3, CD20, CD79a, CD5, CD10, CD23, anti-cyclin D1 and kappa and lambda light chains. Antigen retrieval was performed using either a microwave oven or a pressure cooker. Labelling was performed with an avidin-biotin-peroxidase labelling system. A final diagnosis was reached in 37 out of 41 cases (90%): B-chronic lymphocytic leukaemia (15 cases), follicular lymphoma (10 cases), mantle-cell lymphoma (eight cases) and lymphoplasmacytoid lymphoma/immunocytoma (four cases). In the remaining four cases, a generic diagnosis of low-grade B-cell neoplasm was made. The immunophenotyping of formalin-fixed marrow is a useful technique for diagnosing most of the low-grade B-cell neoplasms.