In order to study the clinical and immunologic effects of a brief course of leukapheresis, 14 patients with clinically similar rheumatoid arthritis were segregated into 2 subgroups. Group A (7 patients) had subnormal lymphocyte tritiated thymidine incorporation to soluble antigens. After a brief course of leukapheresis, 6 of these 7 patients demonstrated substantial clinical improvement associated with significantly enhanced lymphocyte tritiated thymidine incorporation to soluble antigens. In contrast, none of the 7 patients in group B (normal immune functions) demonstrated similar changes in articular indices or lymphocyte proliferation. Thus, clinical improvement induced in a subset of rheumatoid arthritis patients appears to reflect modulation of function, and not simply immunosuppression.