We used a panel of paraffin antibodies to determine whether neoplastic and nonneoplastic lymphoid aggregates in the bone marrow can be distinguished reliably. Formalin-fixed, paraffin-embedded bone marrow core biopsy specimens with lymphoid aggregates were stained using primary antibodies directed against bcl-2, bcl-6, CD5, CD10, CD20, and CD23. We studied 61 cases (26 follicular lymphoma and 35 benign or atypical aggregates). We found that no single stain is sufficient for identification of neoplastic lymphoid aggregates. However, this distinction was made possible by using a panel of antibodies. Under the conditions we tested, the most useful antibodies were CD10, bcl-2, CD5, and CD20. Most benign or atypical aggregates do not express CD10 and CD23. In addition, nonneoplastic aggregates had a large population of T cells. bcl-2 was useful in an architectural context for distinguishing neoplastic aggregates. bcl-6 often was expressed in both neoplastic and nonneoplastic aggregates and, thus, poorly discriminated between these processes. We studied the expression of CD10 and bcl-6 in selected lymph nodes in some cases.