Therapeutic effects of HL for a collagen-induced arthritis (CIA) mouse models of HL-23 composed of 95mol% l-α-dimyristoylphosphatidylcholine (DMPC) and 5mol% polyoxyethylenedodecylether (C 12 (EO) 23 ) in vivo were examined. Remarkably high therapeutic effects of HL-23 for CIA mouse models were obtained on the basis of clinical assessment of arthritis. The reduction of hyperplastic synovial membrane (pannus tissue) and destruction of the cartilage and bone by HL-23 was revealed on the basis of hematoxylin and eosin (HE) and safranin O staining. Furthermore, the downregulation of inflammatory cytokines such as interleukin (IL)-1β, tumor necrosis factor (TNF)-α, and IL-6 for CIA mouse models treated with HL-23 were investigated. Remarkably high therapeutic effects without joint swelling were obtained in CIA mouse models treated with HL-23.