This is a Phase II, randomized, double-blind, parallel-group, placebo-controlled study to evaluate the safety and efficacy of MABT5102A in patients with mild to moderate Alzheimer's Disease.

Inclusion Criteria:

• Diagnosis of probable AD according to the National Institute of Neurological and Communicative Disorders and Stroke and the Alzheimer's Disease and Related Disorder Association (NINCDS-ADRDA) criteria
• Mini-Mental State Examination (MMSE) score of 18-26 points at screening
• Geriatric Depression Scale (GDS-15) score of < 6
• Completion of 6 years of education (or good work history consistent with exclusion of mental retardation or other pervasive developmental disorders)
• If receiving concurrent AD treatment, patient must be on the medication for at least 3 months at a stable dose for at least 2 months prior to randomization.

Exclusion Criteria:

• Severe or unstable medical condition that, in the opinion of the investigator or Sponsor, would interfere with the patient's ability to complete the study assessments or would require the equivalent of institutional or hospital care
• History or presence of clinically evident vascular disease potentially affecting the brain
• History of severe, clinically significant (persistent neurologic deficit or structural brain damage) central nervous system trauma
• Hospitalization within 4 weeks prior to screening
• Previous treatment with MABT5102A or any other therapeutic that targets Abeta
• Treatment with any biologic therapy within 5 half-lives or 3 months prior to screening, whichever is longer, with the exception of routinely recommended vaccinations, which are allowed