There is considerable evidence to suggest that collagen-induced arthritis (CIA) is a Th1-mediated disorder. But the details about the roles of CD4T and CD8T in CIA are still unclear. It was reported that there is a presence of blood CD4+ cells co-expressing CD11b in Rheumatoid Arthritis. In CIA mice, we investigated the blood lymphocyte, which is the most population of the number, by using Flow cytometry. We observed what kind of physiology state develops CD11b+ lymphocyte. In CIA-days 28 and 35, the number of white blood cell was unchanged. In CIA-day 28, when a dim symptomatic sign had begun to develop in limbs, a decline in lymphocyte count was detected because there was a rise in neutrocyte/lymphocyte rate. The decrease of B cells were remarkable, therefore T cells increased relatively. In CIA day28, increase of CD4% was more massive than increase of CD8%, there fore CD4/CD8 rate increased. In Prednisolone 10mg/body, the symptom of arthritis was completely suppressed, but significant decrease of the number of white blood cell and decrease of lymphocyte% persisted till day 35. In addition, in Prednisolone days28 and 35, the CD4/CD8 ratio decreased because increase of CD8 was more massive than increase of CD4. In a CIA group, the onset was remarkable on day 35 and CD11b+ lymphocyte fraction was recognized but Prednisolone suppressed the onset of arthritis and CD11b expression of lymphocyte. [Jpn J Physiol 55 Suppl:S231 (2005)]