Abstract The anti-vitamin B12 properties of four descobalto-corrins isolated from Chromatium cells were compared with vitamin B12-anilide, vitamin B12-monocarboxylic acid, cobinamide, and Friedrich's C′3-2-methyl-2-aminopropanol(1)-vitamin B12 (compound S-102). Among the systems where response to cyanocobalamin was markedly inhibited by the descobalto-corrin additions were the growth response of Escherichia coli (Davis 113-3), Euglena gracilis, and Arthrobacter duodecadis. The inhibition indices obtained with two of the descobalto-corrins were similar to those obtained with Friedrich's S-102, and considerably lower than those noted with vitamin B12-anilide, vitamin B12-monocarboxylic acid, and cobinamide. The growth response of Lactobacillus leichmannii (ATCC 7830) was not affected when descobalto-corrins were added to the cyanocobalamin-supplemented media. In experiments with porcine intrinsic factor preparations, two of the descobalto-corrins were potent inhibitors of vitamin B12 binding, but not as active as hydroxocobalamin, vitamin B12-anilide, or Friedrich's S-102 compound; they were more active than cobinamide and vitamin B12-monocarboxylic acid. The descobalto-corrins were not effective in blocking vitamin B12-binding by human gastric juice or ascitic fluid from mice with Ehrlich ascites tumor.