In the present study we describe a sensitive ELISA for determination of glial fibrillary acidic protein (GFAP). To validate the method combined determinations of GFAP and S-100 protein were performed in cerebrospinal fluid (CSF) of normal children and children with autism. The GFAP ELISA is of sandwich type and uses the biotin-avidin system. Sensitivity was 16 pg/ml. Between-day precision was 0.079 (coeff. of varience). S-100 protein concentrations were measured using a commercially available ELISA kit. Normal CSF from children and young adults were analysed. The CSF levels of GFAP in normal children were low (16–163 pg/ml). Both GFAP and S-100 protein concentrations correlated with age (P < 0.01 and P < 0.05, respectively), but the GFAP increment was more pronounced, probably reflecting the age-dependent expansion of the fibrillary astrocytes in the central nervous system (CNS). GFAP levels in children with infantile autism were higher than those in normal children of the same age range. S-100 protein concentrations were similar in both groups. High levels of GFAP in combination with normal S-100 protein concentrations in CSF indicates reactive astrogliosis in the CNS. In conclusion, the sensitive ELISA described makes it possible to measure low levels of GFAP present in the CSF of children. Combined assays of GFAP and S-100 protein can be used to discriminate between acute and chronic brain disorders in children.