Alternative splicing of the transcript encoding the beta-amyloid precursor protein (BAPP) of Alzheimer's disease produces multiple mRNA species. Translation of these mRNAs predicts protein products of 770, 751, and 695 amino acids. The difference arises from the inclusion in BAPP-770/751 of a 56-residue insert region which is homologous to Kunitz-type protease inhibitors. We have prepared and affinity-purified anti-peptide antibodies that react specifically with either BAPP-770/751 (insert-specific) or BAPP-695 (junction-specific). A detectable level of the mRNA corresponding to the BAPP-770/751 protein was found in all cell lines tested. Immunoprecipitation of 35S-labeled proteins from these cell lines showed them to contain one or two Mr 105,000 bands reactive with the insert-specific serum, i-291. In contrast, only cos-7 cells and the human neuroblastoma cell line, IMR-32, contained mRNA species that encode the BAPP-695 protein, as shown by Northern analysis with a junction-spanning oligonucleotide probe. A band of Mr 95,000 was immunoprecipitated specifically from these two cell lines using the junction-specific serum, J-284. Indirect immunofluorescence labeling of cells corroborated these findings. All cells reacted with the insert-specific antibodies, i-291 and i-324. Only cos-7 and IMR-32 cells reacted with the junction-specific antibody, J-284. These results demonstrate the usefulness of anti-peptide antibodies for the differential detection of the BAPP-695 and BAPP-770/751 proteins.