Cerebrospinal fluid (CSF) samples from 150 patients with multiple sclerosis (MS) and 190 patients with other neurological disorders were examined with a battery of tests (the IgG profile) to detect quantitative and qualitative abnormalities in CSF IgG. The CSF IgG profile consisted of an IgG/albumin (IgG/alb) ratio, an IgG index, and examination by agarose gel electrophoresis (AGE) and isoelectric focusing (IF) to detect oligoclonal bands. The tests were compared for diagnostic accuracy, including false-positive results. The IgG/alb ratio was less reliable in confirming the diagnosis of MS, but no significant difference in accuracy was found among the other three methods. IF tended to identify more possible and probable MS cases than did AGE but gave a higher rate of false-positives. The IgG index and IgG synthesis rates showed no significant difference in their ability to identify MS patients. Steroid administration decreased the incidence of abnormal IgG/alb ratios and IgG indices, but not abnormal oligoclonal bands. Central nervous system (CNS) infections or immunological diseases involving the CNS produced a 28 to 40% incidence of abnormalities in the CSF. Neither the patient's age, sex, duration of illness, activity of disease, nor longitudinal course correlated with the CSF findings. A few (1%) control neurological patients had all components in the CSF IgG profile abnormal. For most routine clinical purposes the IgG index and AGE are sufficient for confirmation of diagnosis, and the IgG index was the best single test in our series.