A simple ELISA method is described for determinations of S-100 protein concentrations in CSF. The assay has a useful range of 200-3200 pmol/l. The precision of the ELISA was estimated using a pool of CSF. The coefficient of variation was 0.18 within assay, 0.17 between assay and 0.17 between day. The S-100 protein is stable in the CSF as no measurable differences in S-100 concentrations were observed in samples stored at room temperature for 2 days. No correlation between age and S-100 concentration was found when determinations were performed in CSF from neurologically healthy males. Furthermore, no changes of S-100 was observed in a lumbocisternal CSF gradient from patients with normal pressure hydrocephalus. Thus, the described ELISA represents an easy to handle and reliable method, well suited for routine determinations of S-100 protein concentration in the CSF.