The occurrence of soluble immune complexes (IC) in the cerebrospinal fluid (CSF) of 14 multiple sclerosis (MS) patients, four acute polyradiculoneuritis patients, 30 patients with other neurological diseases (OND) and 30 patients with disc prolapse (DP) was examined by a solid phase C1q-protein A binding assay (C1q-PABA) and a complement consumption test. IC-positive reactions were observed only in the C1q-PABA. The binding indices determined by the C1q-PABA differed significantly (P < 0.01) when the MS or the OND patient groups were compared to the DP group. No significant (P < 0.1) difference was observed between the indices in the MS and OND groups. Binding indices in C1q-PABA showed no correlation either to IgG concentration, total protein concentration or cell counts in CSF of MS patients. Three of the four polyradiculoneuritis patients were strongly IC-positive while the fourth patient was negative. Filtration and PEG-precipitation data indicated that a major part of the IgG-containing IC in CSF detected by C1q-PABA was of macromolecular nature.