BACKGROUND: Patients on chronic hemodialysis are prone to develop amyloid deposits of misfolded beta(2)-microglobulin (beta(2)M) in osteoarticular tissues. beta(2)M with various deletions/truncations and chemical modifications has been found together with structurally intact beta(2)M in extracts of beta(2)M amyloid fibrils. The state of the circulating population of beta(2)M molecules has not been characterized previously with high-resolution methods. METHODS: We used immunoaffinity-liquid chromatography-mass spectrometry analysis of serum samples to examine whether structurally modified beta(2)M is generated in the circulation. In addition, we developed an immunoassay for the quantification of a cleaved beta(2)M variant in biological fluids based on novel monoclonal antibodies and applied this assay to patient and control sera. RESULTS: A specific alteration compatible with the generation of lysine-58-cleaved and truncated beta(2)M (DeltaK58-beta(2)M) was found in the sera of many (20%-40%) dialysis patients but not in control sera or sera from patients with cerebral amyloidosis (Alzheimer disease). Applied to patient sera, specific immunoassays revealed that dialysis, as expected, significantly lowered the total beta(2)M concentration, but the concentrations of DeltaK58-beta(2)M remained unchanged after dialysis. The results also show that patients dialyzed with less biocompatible membranes have higher serum concentrations of cleaved beta(2)M (mean, 8.5, 1.8, and 0.7 mg/L in cuprophane membrane-dialyzed, polysulfone membrane-dialyzed, and control sera, respectively). CONCLUSIONS: This study for the first time demonstrates and assigns the structure of a specific beta(2)M variant in sera from dialysis patients. Because this variant is conformationally unstable in vitro, it may be involved in in vivo amyloidogenesis.