Substantial amounts of vitamin B₁₂ were found in some edible algae (green and purple layers) and algal health food (chlorella and spirulina tablets) using the Lactobacillus delbrueckii subsp. lactis ATCC7830 microbiological assay method. Corrinoidcompounds were purified and characterized from these algae to clarify the chemical properties and bioavailability of the algal vitamin B₁₂. True vitamin B₁₂ is the predominate cobamide of green and purple layers and chlorella tablets. Feeding the purple layer to vitamin B₁₂-deficient rats significantly improved the vitamin B₁₂ status. The results suggest that algal vitamin B₁₂ is a bioavailable source for mammals. Pseudovitamin B₁₂ (an inactive corrinoid) predominated in the spirulina tablets, which are not suitable for use as a vitamin B₁₂ source, especially for vegetarians.