Interleukin 10 polarizes macrophages toward the M1 phenotype and inhibits M1 marker expression in mice with collagen-induced arthritis. Joint macrophages from interleukin 10–knockout (IL-10−/−) mice and wild-type (WT) mice with collagen-induced arthritis (CIA) on day 45 after chicken type II collagen (CII) immunization were isolated. Cells positive for macrophage marker F4/80 detected by flow cytometry were defined as macrophages. Within that population, macrophages were analyzed using inducible nitric oxide synthase (iNOS) as a marker of the classically activated macrophage (M1) phenotype and CD206 as a marker of the alternatively activated macrophage (M2) phenotype. (A) Percentage of M1 macrophages (F4/80+iNOS+) and M2 macrophages (F4/80+CD206+) of IL-10−/− mice and WT mice with CIA assessed by flow cytometry. (B) and (C) Total M1 and M2 cells were quantified. (D) The total cellular RNA was extracted and analyzed for the expression of genes associated with M1 and M2 macrophage markers by quantitative RT-PCR. Data are mean ± SEM (n = 3). *P < 0.05; **P < 0.01. NS = Not significant.