Twenty-one patients with definite active multiple sclerosis (MS) were treated with a monoclonal anti-T CD4/B-F5 (murine IgG1) antibody for 10 days. Side effects were observed in 11 patients during the first infusion. These side effects were accompanied by and probably related to a transient increase in IL6 and TNF alpha serum levels. This problem led to treatment interruption in one patient. Neither clinical improvement nor deterioration was observed in the course of treatment. EDSS improvement (> 1 point) occurred in six patients one month post-treatment. One month after the end of treatment total lymphocytes and CD3+ and CD4+ cells were significantly decreased. Cytokine analysis performed in serum and in CSF before and after treatment showed no induced modifications. Ten patients developed xenogenic antibodies. It is of interest that the patients with relapsing-remitting forms were relapse-free at the 6th month post-therapy.