Previously, we uncovered evidence of an infectious agent, Chlamydia pneumoniae, associated with sporadic Alzheimer's disease (AD). We demonstrated that C. pneumoniae was present in 90% of brain materials examined from AD patients, compared with 5% in age-matched, non-AD patients. We further showed that microglia and astroglia in the CNS are host cells for the bacterium. RT-PCR analyses indicated that the bacterium is metabolically-active in the brain and could be cultured in a human monocyte cell line (THP-1). Importantly, we showed that the organism is concentrated in areas of AD-neuropathology also containing tau and beta-amyloid. The present study was designed to investigate the effects of C. pneumoniae infection on beta amyloid production in human monocytes (THP-1), astroglial (CCF-STTG1), epithelial (Hep 2), and endothelial (HBMEC) cell lines. We investigated the host pathogen relationship with C. pneumoniae and whether entities characteristic of AD neuropathology such as betaamyloid are associated directly with this infectious agent.