We report a study of lymphocyte subsets in experimental arthritis induced in Wistar Furth rats by native human type II collagen and muramyl dipeptide. This experimental arthritis shares similarities with both the spondyloarthropathies and rheumatoid arthritis. Peripheral blood T lymphocytes, primarily the CD4+ cells (p = 0.01), were lower in arthritic rats than in the controls, although the difference in the CD4/CD8 ratio was not statistically significant. Splenic CD4 cells were significantly (p = 0.03) more numerous in arthritic rats, while the numbers of MHC class II positive cells (p = 0.002) and kappa-bearing B-cells (p = 0.0004) were significantly lower. Determination of peripheral blood and spleen lymphocytes subsets could therefore be used for the assessment of arthritis and for the evaluation of therapeutic agents. Thymic T-cell differentiation does not appear to be impaired in this model. These results differ from the peripheral blood disturbances described in the active stages of human rheumatoid arthritis and are more similar to those reported in ankylosing spondylitis patients. However, the absence of alterations in the Peyer's patches suggests that pathogeneic mechanisms involving mucosal areas and exogenous intestinal antigens are not reproduced in this model.