The light chain composition of MS and SSPE CSF oligoclonal IgG bands was examined using isoelectric focusing and a sensitive peroxidase-antiperoxidase (PAP) staining technique specific for gamma heavy chains (gamma), kappa light chains (k), or lambda light chains (lambda) and a radioimmunoassay (RIA) for gamma, k or lambda. Many bands in the 7 MS and 4SSPE CSF examined were monoclonal, staining for either IgG-k or IgG-lambda. By staining, all MS CSF were k predominant; SSPE CSF were variously k or lambda predominant. RIA confirmed the k predominance of MS CSF. Three MS and 2 SSPE CSF contained bands staining for lambda alone, i.e. free light chains. Analysis of RIA data confirmed these findings in 2 MS cases. The difference in light chain predominance of MS and SSPE CSF may reflect differences in the antigenic target, or the age of patient at the time when band-synthesizing clones are triggered. Six of 7 MS and all 4 SSPE CSF contained oligoclonal bands staining for gamma and for both kappa and lambda, probably representing artifacts of IEF. No predominant immunochemical differences between bands in MS and SSPE were detected.