Amyloid β (Aβ) protein is a soluble 4 kD protein, which is characteristically deposited in the Alzheimer's disease (AD) brain. Mixed fetal guinea pig brain cultures secrete large amounts of Aβ with the same sequence as human Aβ. Thus, this system is well suited for the study of perturbations that might play an important role in amyloid deposition. In this study we used mixed primary cultures from the fetal guinea pig brain that were maintained for many weeks in medium containing 5% calf serum and then treated with increasing concentrations of hydrogen peroxide to investigate the effect of oxidative stress on Aβ metabolism. Following treatment with hydrogen peroxide, we analyzed Aβs ending at Aβ40 or Aβ42(43) by sandwich-ELISA, immunocytochemistry, and immunoprecipitation. Aβ was secreted in large amounts into the medium of these cultures, and, in agreement with previous reports, 90 % of secreted Aβ was Aβ1-40 and 10 % was Aβ1-42. As hydrogen peroxide concentration was increased, Aβ secretion first increased slightly and then decreased dramatically. Control cultures showed essentially no staining with 4G8 (specific for Aβ17-24), BA-27 (specific for Aβ ending at Aβ40), or BC-05 [specific for Aβ ending at Aβ42(43)]. Following treatment with high concentration hydrogen peroxide, many cells with neuronal morphology stained positively with 4G8 and BC-05 but not with BA-27. This suggests that oxidative stress results in selective deposition of Aβ ending in 42(43), a result of considerable interest because this peptide deposits selectively in senile plaques early in the course of AD. We are currently biochemically analyzing the Aβ deposited in these cultures after hydrogen peroxide treatment to confirm its precise nature.