Arthritis was induced in DBA/1 mice by passive transfer of syngeneic anti‐type II collagen (CII) scrum concentrate. After transfer of serum containing 0.2 or 0.5 mg anti‐CII auto‐antibodies the first clinical signs of arthritis appeared 48 h after injection. Severe clinical arthritis was detected 96 h after injection. Immunohistochemical analyses of joints 48 h after serum injection revealed synovial foci in intercarpal and mclacarpophalangeal joints of macrophage‐like cells, expressing C3bi‐receptors and major hislocompatibility complex class II molecules, and infiltration of few CD4+ lymphocytes. Later (96 h afler injection), the inflamed synovia were dominated by C3bireceptor+ polymorphonuclear cells. In contrast to conventionally induced collagen arthritis (CIA), the inflammatory infiltrates, filling joint spaces and synovial tissue, were extensively dominated by polymorphonuclear cells, whereas macrophage‐like cells expressing class II molecules and a few T cells were seen only in the periphery of the developing pannus. The anti‐CII serum induced arthritis may be used as a model for studies of humoral mediated mechanisms operating in conventionally induced CIA as well as in rheumatoid arthrilis.