We present our first experience with the determination of free kappa light chains (FKLC) in cerebrospinal fluid and serum by ELISA and a demonstration of their oligoclonality by means of isoelectric focusing and affinity immunoblotting. Using FKLC quotient Q~.c (ratio CSFFioc:Serum~,c) and FKLC index (QFxLC:Qalbumin), the intrathecally synthesized FKLC fraction could be estimated. Elevated QFio c was found in all 13 multiple sclerosis patients and in 2 from 17 patients with other diseases. An elevated FKLC index as the most reliable quantitative indicator of intrathecal FKLC synthesis was found in all multiple sclerosis patients and in none of the patients with other diseases. CSF-restricted oligoclonal FKLC could be convincingly demonstrated in all 13 multiple sclerosis patients and in none of the patients with other diseases. Although the number of patients examined was small, we can conclude tkat both an elevated FKLC index and/or CSF-restricted oligoclonal FKLC may strongly support the clinical diagnosis of multiple sclerosis. Unlike IgG, demonstration of CSF-restricted oligoclonal FKLC does not seem to be more sensitive for the laboratory support of the diagnosis of multiple sclerosis than the FKLC index. Since isoelectric focusing followed by affinity immunoblotting is much more laborious than ELISA, it will be probably used in specialized laboratories ohly, mainly for research purposes. On the contrary, the ELISA method described here is well-suited for routine laboratories dealing with CSF analysis.

        Key words: cerebrospinal fluid, free kappa light chains, multiple sclerosis.