Perturbations in T cells and T cell subsets of peripheral blood lymphocytes were looked for, using monoclonal antibodies, in nine patients with rheumatoid arthritis (RA) and four patients with systemic lupus erythematosus (SLE). All SLE patients were in an acute phase of their disease, but had not yet received steroids. Seven of the nine RA patients presented an active illness, recently diagnosed in five cases, and received no steroids nor D-penicillamin. T cell subsets alterations responsible for abnormal values of the OKT4 +/OKT8 + immunoregulatory ratio, were improved by in vitro incubation of the lymphocytes with synthetic thymulin in eight out of nine RA patients. No significant modification occured for SLE patients' lymphocytes. These results support the possible beneficial role of thymulin in the treatment of rheumatoid arthritis.