It has been reported that serum vitamin B12 levels assayed by saturation analysis methods may give misleadingly high results, so much so that the diagnosis of vitamin B12 deficiency may be obscured. This defect was ascribed largely to assays using a vitamin B12 binder other than pure intrinsic factor. To test out this hypothesis two assays were set up, one using saliva (non-intrinsic factor R-binder) and the other using human gastric (intrinsic factor) as B12-binding agents. Both assays were able to differentiate sera from patients with pernicious anaemia from those from control subjects. Published results accumulated over the past 10 years indicate that properly designed and performed saturation analysis vitamin B12 assays are as reliable as microbiological assay methods for detecting low serum B12 levels. The failure of some methods to do does not appear to be due to the nature of the B12-binding agent.