We compare serum proteins from rainbow trout, chinook salmon, coho salmon, and oyster toadfish with intrinsic factor as binding proteins in a simplified radioassay for B12. Regression analysis of B12 values, determined in 21 serum samples, shows good correlation (r greater than .975) between results for the fish sera and intrinsic factor. The accuracy of the five assays, as evaluated by analytical recovery of B12 added to pooled human serum, ranges from 90 to 110%. Intra-assay precision ranges from 2.6% for coho salmon serum to 5.5% for intrinsic factor, Ionic strength and variations in pH influence binding of [57Co]vit B12 to the fish sera. Maximum binding occurs from pH 6 to 10 at an ionic strength of 0.1 for all sera. The sera are stable for longer than two years when stored at -20 degrees C. Important advantages of fish sera are their high binding capacity (typical assay dilutions range from 1500-fold for trout serum to more than 50 000-fold for chinook salmon); high affinity for B12 (K greater than 10(12) liter/mol); their relative constant binding characteristics as compared to commercial intrinsic factor preparations; and the finding that the accuracy of radioassays with use of fish sera is not significantly affected by the amount of B12 or human serum proteins present.