Production of the lymphokine LIF (leukocyte migration inhibitory factor) by normal lymphocytes on stimulation with mitogens is inhibited by E type prostaglandins (PGE). It has been reported that the leukocytes of patients with multiple sclerosis (MS) are resistant to this inhibitory effect. We have studied 10 MS patients using an indirect technique for measuring LIF activity. This allows separation of the effects of PGE on lymphocyte LIF production from the effects on the action of LIF on the target granulocytes. There was no difference between normal subjects and MS patients in the effects of prostaglandins E1 and E2 in concentrations of 2.5 and 0.25 μg per milliliter, on LIF production, or on granulocyte response to LIF. These results do not support the hypothesis that the chronic inflammatory process in MS is the result of resistance to the normal inhibitory effects of PGE on the cellular immune response.