In 33 cases of multiple sclerosis treated with ACTH (2500 u during 40 days) IgG and albumins were determined in the serum and cerebrospinal fluid by Mancini's method. These determinations served for calculation of the IgG: albumin ration and IgG index. After the treatment a significant reduction was found of the serum level of IgG and IgG index, without significant changes in albumin concentration. The effect of ACTH therapy on IgG synthesis within the CSF spaces was not significant in the whole group. In the subgroup of patients with initially demonstrated IgG synthesis within the CSF spaces (before the treatment) a significant decrease was found of the IgG index and a non-significant decrease of the absolute IgG level in the CSF. A comparison of the results with our previous investigations shows that ACTH, despite its positive effect on multiple sclerosis exacerbations exerts a much weaker effect on the local IgG synthesis in the central nervous system than high doses of prednisone, despite an evident reduction of serum IgG level by ACTH.