IL-3-like activity (IL-3-LA) is a growth factor that stimulates stem cell maturation. We examined the production of IL-3-LA by peripheral blood mononuclear cells of 10 patients with relapsing-remitting multiple sclerosis (MS) following one year of treatment with intravenous immunoglobulins (IVIG). The results were compared with those obtained in 13 age- and sex-matched untreated patients with relapsing-remitting MS and in 14 healthy controls. IL-3-LA was assayed using the IL-3-dependent 32-D-cl-23 murine cell line. IL-3-LA production was 60% higher in untreated MS patients than in healthy controls (134±19 u/ml and 78.7±15.9 u/ml, respectively; p<0.01), and lower in patients treated with IVIG than in untreated patients (101.4±4.9 u/ml; p<0.02). IL-3-LA production also decreased after incubation of mononuclear cells with IVIG of both untreated MS patients and controls. This study indicates a possible beneficial effect of IVIG on the immunological status of MS patients.