Diverse neurotoxic insults results in proliferation and hypertrophy of astrocytes. The hallmark of this response is enhanced expression of the major intermediate filament protein of astrocytes, glial fibrillary acidic protein (GFAP). These observations suggest that GFAP may be a useful biomarker of neurotoxicity. To investigate this possibility, the authors administered prototype neurotoxicants to experimental animals and assessed the effects of these agents on the tissue content of GFAP, as determined by radioimmunoassay. A review of the background, design and results of these experiments are presented in the paper. Study findings indicate that radioimmunoassay of GFAP is a sensitive and specific biomarker of neurotoxicity.