Suppressor cell function has been previously reported to be decreased in patients with progressive multiple sclerosis (MS). The abnormality could not be corrected in vitro and was present even after patients were treated with immunosuppressive agents. We now report that interferon beta augments suppressor cell function in vitro in progressive MS. Nonspecific suppressor cell function as measured in a concanavalin A (Con A) suppressor assay was reduced in 24 MS patients (mean percent suppression, 19.6 +/- 2.2) when compared to 19 normal subjects (mean percent suppression, 35.0 +/- 3.3). The data are highly significant (p less than 0.001). When recombinant human interferon beta (10(3) units/ml) was added to lymphocyte cultures with Con A, suppressor activity improved significantly. The mean percent suppression improved from 19.6 +/- 2.2 to 37.8 +/- 2.6 in MS (p less than 0.001) and from 35.0 +/- 3.3 to 46.2 +/- 3.5 (p less than 0.025) in control subjects. This study shows that recombinant interferon beta improves suppressor function in humans, an effect that is particularly significant in progressive MS.