Studies were undertaken on the processing of Alzheimer's disease-associated beta-amyloid precursor protein in normal cultured human fibroblasts and a human neuroblastoma cell line. Major differences in processing between the secreted and intracellular forms of the precursor were found. The intracellular form appears to undergo amino-terminal processing yielding many smaller fragments, whereas the secreted form does not show any further proteolytic cleavage after its release from the cell surface. In pulse-chase experiments, antibodies to the A4 region immunoprecipitated bands of Mr = 92,000-128,000, which represent the intact precursor; several smaller intracellular fragments of Mr = 70,000-72,000, 55,000, 33,000 and 6,000 also immunoprecipitated with this antibody. The Mr = 6,000 band cleared from the cell very quickly and is postulated to be the A4-carrying remnant of the secreted protein. The data show that a fragment of Mr = 33,000, which includes the A4 region, is one stable processed end-product of the intracellular precursor protein. It is possible that different posttranslational modifications are the signals responsible for the differences in processing between the secreted and intracellular amyloid precursor protein.