In vitro cell culture model systems for investigating the biochemical mechanisms involved in the neurodegenerative actions of beta-amyloid peptide (beta-AP) have been established. Using rat pheochromocytoma PC12 or human epitheloid HeLa cell lines, submicromolar concentrations of the beta-AP fragments beta 1-40, beta 1-39, and beta 25-35, but not beta 1-28, were found to inhibit the reduction of the redox dye 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT). In both cell lines, the beta-AP-sensitive component represented approximately 70% of total cellular MTT reduction. When the reduction of a series of structurally related dyes was compared with that of MTT, the reduction of 3 alpha-naphthyl-2-phenyl-5-(4-nitrophenyl)-2H-tetrazolium chloride (NTV) was also found to be sensitive to beta 25-35, but that of seven other redox dyes was not. A property common to MTT and NTV is that they are both readily taken up into PC12 and HeLa cells and do not require an artificial electron coupling agent to be reduced. Microscopic analysis of MTT-formazan product formation in PC12 and HeLa cells following beta 25-35 treatment revealed that it was the intracellular component of the reduction of this dye that was abolished. These results support the hypothesis that the cellular reduction of MTT represents a specific indicator of the initial events underlying the mechanism of beta-AP toxicity.