Chronic inflammatory infiltrate in B6 mice with collagen-induced arthritis. Histological assessment of arthritis was carried out in early arthritis (up to 2 weeks after onset, n = 8) and late arthritis (6 to 8 weeks after onset, n = 8). (a) Severe joint destruction with massive accumulation of polymorphonuclear cells (PMNs) was observed in DBA/1 in early arthritis. (b) In B6 mice, the infiltrating cells were predominantly mononuclear in early arthritis and there was less joint erosion. (c) In late arthritis, the inflammatory response largely resolved in DBA/1 mice, although the joint destruction was not reversed. (d) The inflammatory response remained active in B6 mice in late arthritis and there was progressive joint erosion. Original magnification, × 100. (e) The numbers of joints showing foci of inflammatory cells (lymphocytes and PMNs) in the joint were compared in early and late arthritis. **P < 0.01.