The inflamed synovium of a patient with RA is intensively infiltrated with lymphocytes which often form follicles with aparent germinal centers. The synovial fluid contains aggregates of IgG and its complexes with rheumatoid factor. These evidences point out the immunological significance for the synovial inflammation and also suggest the need to determine whether cell-mediated immunity is concerned with the pathogenesis of RA. In the present study, an attempt has been made to determine whether heat-aggregated IgG and synovial crude extract have acted as antigns capable of causing cellular hypersensitivity in patients with RA. MIT and LMIT have been taken as in-vitro expression of cellular hypersensitivity. The value below the mean percent migration of normal subjects minus 2 SD was considered to be positive. MIT with heat-aggregated IgG was positive in 6 out of 14 patients with RA. Heat-aggregated IgG and synovial crude extract inhibited leucocyte migration in 4 out of 12 patients with RA. Indirect LMIT with heat-aggregated IgG was positive in 4 out of 14 patients with RA and with synovial crude extract in 5 out of 14 patients. None of normal subjects showed positive leucocyte migration inhibition to both antigens. There was no correlation between positive tests and titers of rheumatoid factor, duration, activity of the disease. The data presented above raise the possibility that cell-mediated immune mechanisms play a role in the pathogenesis of RA.