Amyloid β-protein (Aβ) and secreted β-amyloid precursor protein (sAPP), derived from β-amyloid precursor protein (APP), are normally released by cultured mammalian cells. We investigated by pulse-chase analysis the secretion kinetics of these two APP derivatives using mouse cholinergic SN49 cell lines stably transfected with mouse APP695 cDNA. After both Aβ and sAPP peaked at about the second hour, sAPP decreased with a half-life of approximately 5 hours, but Aβ remained almost unchanged for at least 14 hours. These results indicate that Aβ is more stable than sAPP in the SN49 conditioned medium. © 1995 by Academic Press, Inc.