The case history is presented of a patient with multiple sclerosis who developed acute polyradiculoneuritis 11 days after subcutaneous implantation of a swine brain preparation. By means of lymphocyte transformation tests (LTT), sensitization against brain gangliosides could be demonstrated 16 days after the implantation. A second patient who underwent the same treatment showed neither clinical symptoms nor sensitization against brain gangliosides in the LTT. Patients did not show reactivity when tested with myelin basic protein. The polyradiculoneuritis was caused by the immune response to the implanted swine brain cross-reacting with human nervous system gangliosides.