Arthritis induced with type II collagen in DBA/1 mice, was analyzed by immunohistochemical techniques. In the earliest detectable pathologic changes, before any macroscopic signs, an accumulation of Mac1+, macrophage-like cells, and an increased expression of major histocompatibility class II antigens were observed focally in the synovial lining layer. In these foci, CD4+ and interleukin 2 receptor expressing T lymphocytes were regularly detected, but not usually other sets of lymphocytes such as B lymphocytes and CD8+ lymphocytes. In clinically detectable arthritis, there was a prominent infiltration of Mac1+ cells, both polymorphonuclear-like and macrophage-like cells. T cells were relatively few, suggesting that they do not play a primary effector role, but rather that they may regulate or permit the self-perpetuative inflammation.