Cell lines transfected with the Swedish Alzheimer's disease amyloid precursor protein APP670/671 mutation release significantly more beta-amyloid than wild-type cells. Citron et al. [Proc. Natl. Acad. Sci. USA (1994) in press] have recently shown that fibroblasts carrying the APP670/671 mutation also release more beta-amyloid than control cells [1]. The present study confirms a ca. threefold increase in beta-amyloid release from mutation-bearing fibroblasts. APP mRNA levels did not differ between mutation-bearing and control cells, although mutation-bearing fibroblasts contained significantly more APP751/770 than controls. Mild stress decreased beta-amyloid secretion and increased APP751/770 levels in all cell lines. In conclusion, the proportion of APP committed to amyloidogenic processing is increased in fibroblasts from family members with the APP670/671 mutation, and this mutation may also compromise the APP stress response.