Human amyloid A protein (AA) is usually composed of the NH2-terminal 76 amino acid residue of serum amyloid A protein (SAA), although lower and higher molecular weight fragments have been reported. We studied the primary structure of six AA proteins with molecular weights of 11 kDA-15kDA, as determined by SDS-PAGE. Automated Edman degradation of the intact purified proteins and sequence analysis of enzymatic peptides revealed that the AA proteins were composed of only 74 to 87 residues. Moreover, fragments of apolipoprotein E or histones 2a, 3 and 4 were associated with these AA molecules. Thus, AA heterogeneity may reflect diverse processing of the SAA precursor and a very close association with other proteins.