1. beta-Amyloid precursor protein cross-reactive polypeptides were detected in the membrane extracts of a mouse neuroblastoma cell line, NB41A3. Four immunoreactive polypeptide bands were observed on western blots of a cell membrane extract. Their molecular weights as estimated by polyacrylamide gel electrophoresis ranged from 89.1 to 41 kDa. 2. After heparin affinity chromatography, two of these polypeptides strongly cross-reacted with an antibody that recognizes Alzheimer beta-amyloid precursor protein. 3. From the heparin binding fraction, these protein were further separated by reverse-phase high-performance liquid chromatography. A cross-reactive protein was isolated.