This study assessed the relationship between occupational magnetic field exposure, the urinary melatonin metabolite 6-hydroxymelatonin sulfate (6-OHMS), and concentrations of blood-borne soluble amyloid β (Aβ), a protein associated with the hallmark lesions of Alzheimer's disease (AD). Blood and urine samples were obtained from male electric utility workers (n = 60) to quantify two lengths of the protein in plasma, Aβ(amino acids 1-40) and Aβ(1-42), and the urinary concentrations of 6-OHMS. Average Aβ levels were positively associated with categories of magnetic field exposure, but this relationship was weak and did not achieve statistical significance. The melatonin metabolite was inversely correlated with Aβ(1-42) and the ratio of Aβ(1-42) to Aβ(1-40). This observation is consistent with recent in vitro data and provides a plausible mechanism for the association between magnetic field exposure and AD that has been observed in some studies.