Twenty-five defined severe RA patients (pts) (17 F, 8 M) were treated in an open study with a CD4 murine monoclonal antibody (Mab) (B-F5 clone, IgG1). Mab's daily dose was 10 mg (1 pt), 15 mg (2 pts), 20 mg (17 pts), 30 mg (4 pts) and 50 mg (1 pt) for 10 days. Tolerance was fair. Clinical improvement occurred during treatment period or within the first month in all but 2 patients, irrespective of Mab dosage. Improvement duration was variable (1 to 12 months), half of the patients still show signs of improvement at month 4. Biological parameters (CRP) improved parallel to the clinical. At day 180, 25% of the patients showed a reduction of 50% or more of the initial CRP values. There is no modification of RF titers, renal and hepatic parameters. Sequential evaluation showed a decrease of B, TCD3, CD4, CD8 lymphocytes and monocytes two hours after Mab infusion and return to baseline in 20 hours. Xenogenic immunization occurred in 6 patients without influence upon clinical response. These modifications are moderate and transient and do not account for the more prolonged effect in some cases, nor do they offer any prediction of further clinical response.