We used in situ hybridization to assess total beta amyloid protein precursor (beta APP) mRNA and the subset of beta APP mRNA containing the Kunitz protease inhibitor (KPI) insert in 11 Alzheimer's disease (AD) brains and 7 control brains well matched for age and postmortem interval. In AD, we observed a significant two-fold increase in total beta APP mRNA in nucleus basalis and locus ceruleus neurons but not in occipital cortical neurons, hippocampal subicular neurons, or neurons of the basis pontis. The increase in total beta APP mRNA in locus ceruleus and nucleus basalis neurons was due exclusively to an increase in KPI-free beta APP mRNA. We conclude that increased production of KPI-free beta APP in nucleus basalis and locus ceruleus neurons may contribute to the deposition of cerebral amyloid that occurs in AD. To identify the beta APP, we prepared antisera to synthetic peptides corresponding to the carboxyl-terminus (anti-C1), to a near amino-terminal domain (anti-beta APP45-62), and to the KPI domain (anti-KPI36-48). We established that these antisera detect the beta APP by showing that they specifically detect proteins that are markedly augmented in cells transfected with beta APP expression constructs. Anti-beta APP45-62 specifically labels (i) a set of approximately 110-135 kDa membrane-associated brain proteins previously detected by antisera to the carboxyl-terminus of the beta APP (anti-C1) and (ii) soluble approximately 105 and approximately 125 kDa proteins not detected by anti-C1. Anti-KPI36-48 specifically labels the two largest membrane-associated forms of the beta APP and the soluble approximately 125 kDa derivative, but does not label the two smallest membrane-associated forms or the approximately 105 kDa soluble derivative. Anti-beta APP45-62 and anti-C1 both specifically stain senile plaques. This finding suggests (i) that full-length beta APP is present in senile plaques and (ii) that proteolytic processing of the beta APP into insoluble amyloid fibrils occurs, at least in part, locally at the sites of amyloid deposition in AD brain. Analysis of cerebrospinal fluid (CSF) showed that the soluble KPI-containing (approximately 125 kDa) and KPI-free (approximately 105 kDa) derivatives present in brain are readily detected in CSF from both AD cases and controls.(ABSTRACT TRUNCATED AT 400 WORDS)