Serial section analysis of pseudofollicles in B-CLL bone marrow. Frozen tissue sections of a bone marrow trephine biopsy with heavy CLL infiltration were cut on slides covered with adhesive. All sections were subjected, before immunostaining, to antigen retrieval and developed using a sensitive avidinstreptavidin-peroxidase technique and standardized procedures. The monoclonal antibodies included anti-Bcl-2 (Ab124; Dako, Glostrup, Denmark), anti-Ki-67 (MIB-1; Dako), CD3 (Dako), and anti-CD40L (clone TRAP1; Pharmingen-Becton Dickinson, San Jose, CA). Virtually all cells are intensely Bcl-2⁺; pseudofollicles contain numerous Ki-67⁺ (ie, proliferating) elements that are interspersed with numerous CD3⁺ T cells, some being CD40L⁺. Original magnification for Bcl-2 and Ki67, × 250; for CD3 and CD40L, × 100.