alpha-Synuclein (alphaSN), also termed the precursor of the non-Abeta component of Alzheimer's disease (AD) amyloid (NACP), is a major component of Lewy bodies and Lewy neurites pathognomonic of Parkinson's disease (PD) and dementia with Lewy bodies (DLB). A fragment of alphaSN termed the non-Abeta component of AD amyloid (NAC) had previously been identified as a constituent of AD amyloid plaques. To clarify the relationship of NAC and alphaSN with Abeta plaques, antibodies were raised to three domains of alphaSN. All antibodies produced punctate labeling of human cortex and strong labeling of Lewy bodies. Using antibodies to alphaSN(75-91) to label cortical and hippocampal sections of pathologically proven AD cases, we found no evidence for NAC in Abeta amyloid plaques. Double labeling of tissue sections in mixed DLB/AD cases revealed alphaSN in dystrophic neuritic processes, some of which were in close association with Abeta plaques restricted to the CA1 hippocampal region. In brain homogenates alphaSN was predominantly recovered in the cytosolic fraction as a 16-kd protein on Western analysis; however, significant amounts of aggregated and alphaSN fragments were also found in urea extracts of SDS-insoluble material from DLB and PD cases. NAC antibodies identified an endogenous fragment of 6 kd in the cytosolic and urea-soluble brain fractions. This fragment may be produced as a consequence of alphaSN aggregation or alternatively may accelerate aggregation of the full-length alphaSN.