The 4-kilodalton (39 to 43 amino acids) amyloid β protein (βAP), which is deposited as amyloid in the brains of patients with Alzheimer's disease, is derived from a large protein, the amyloid β protein precursor (βAPP). Human mononuclear leukemic (K562) cells expressing a βAP-bearing, carboxyl-terminal βAPP derivative released significant amounts of a soluble 4-kilodalton βAPP derivative essentially identical to the βAP deposited in Alzheimer's disease. Human neuroblastoma (M17) cells transfected with constructs expressing full-length βAPP and M17 cells expressing only endogenous βAPP also released soluble 4-kilodalton βAP, and a similar, if not identical, fragment was readily detected in cerebrospinal fluid from individuals with Alzheimer's disease and normal individuals. Thus cells normally produce and release soluble 4-kilodalton βAP that is essentially identical to the 4-kilodalton βAP deposited as insoluble amyloid fibrils in Alzheimer's disease.