Twenty-one patients with active multiple sclerosis (16 progressive and 5 recurrent forms) were treated with murine monoclonal anti-T CD4/BF5 antibodies in an open trial. Tolerance was relatively good; 11 patients had side effects including facial swellings, skin eruptions and trembling which occurred only at the first infusion. Treatment had to be stopped in one female patient. Nine months after initiating treatment, no relapse had occurred in any of the 5 patients with a recurrent form and improvement was observed in 3 patients with a progressive form. At the end of the treatment period, there was a clear drop in the number of CD3+ cells and particularly in CD4+ cells, a decrease which was less after one month. All the patients who had side effects showed an increased level of serum IL6 and alpha TNF. These findings demonstrate that this type of long-term treatment is possible in patients with multiple sclerosis and suggest that in another trial it would be important to evaluate the effect in recurrent forms.