Detailed protocols are presented for assaying glial fibrillary acidic protein (GFAP), an astrocyte localized protein which serves as a quantitative marker of toxicant-induced injury to the central nervous system. Two different solid-phase assay procedures are described: (1) a nitrocellulose based slot-immunobinding assay and (2) a novel microtiter plate based sandwich ELISA. The performance of both assays was assessed by measuring the content of GFAP in homogenates of specific regions of the rat brain and in homogenates of brain regions damaged by the prototype neurotoxicants, trimethyltin (TMT) and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Both procedures gave similar results that were consistent with previously published observations. By comparing the simplicity, cost effectiveness, safety and speed of the two methods, it appears likely that the sandwich ELISA has several advantages over slot-immunobinding assays.