Comparisons of the 3 previously used methods (continuous flow centrifugation, Millipore®filtration, and biodialysis) for making water samples seston‐free before analysis for cobalamins point to several likely sources of error: (a) unpredictable occlusion of pores in Millipore®membranes during filtration; (b) irreversible adsorption of cobalamins by Millipore®membranes; (c) probable cell injury and leakage by centrifugation. Some water samples gave good agreement on soluble vitamin B12 by all 3 methods, while other samples had appreciable discrepancies. The validity of total vs. sestonic cobalamin determinations therefore is questioned. One lake had time‐dependent fluctuation in soluble cobalamin. Copyright © 1969, Wiley Blackwell. All rights reserved
