Metabolically active B12 analogs and inactive B12 analogs were measured in plasma, red blood cells (RBC), and pooled pernicious anemia serum. B12 values by Lactobacillus leichmannii, Escherichia coli, Euglena gracilis, and radioisotope dilution method (RIDA) as assays for total B12 (active analogs + inactive analogs) were compared to Ochromonas malhamensis values as index of only metabolically active B12. B12 values above those with O malhamensis distinguished inactive analogs from active B12. Inactive analogs contribute 85, 97, 135, and 163% above active B12 activity in normal plasma when E gracilis, L leichmannii, RIDA, and E coli, respectively, were used for B12 analysis. RIDA B12 determinations for active B12 in plasma showed that 44% of the B12 measured was still due to inactive analogs when compared to O malhamensis B12 activity. Inactive B12 analogs contributed 21, 151, and 224% above O malhamensis active B12 in RBC when E gracilis, L leichmannii, and E coli, respectively, were used.