Canine and porcine cerebrospinal fluid (CSF) were fractionated by size exclusion chromatography and analysed by a luminescence enzyme linked immunosorbent assay (ELISA) configured to detect β-amyloid. A peak of activity was observed in the CSF consistent with the molecular weight of β-amyloid. When CSF contaminated with blood was analysed an additional peak of immunoreactivity at a higher molecular weight was observed. The peak of activity was found to be derived from cross-reactivity of the immunoglobulins employed in the ELISA with haemoglobin. These findings are discussed with reference to primary and structural sequence homology between β-amyloid and haemoglobin from a number of species, the known properties of β-amyloid and recent clinical reports.