Four cobalamines (methyl-, hydroxy-, adenosyl- and cyancobalamines) are considered as natural forms of vitamin B12 in human and animal tissues. Methyl- and adenosylcobalamines are the coenzymes of more than 10 enzymes, catalyzing important reactions of lipid, carbohydrate and protein metabolism. The four natural forms of vitamin B12 are interconverted in presence of corresponding enzymatic systems. Content of individual forms of cobalamines and of corresponding coenzymes depends on the function of enzymatic systems involved in their synthesis as well as on the enzymes, which use these derivatives as coenzymes. Spectra of cobalamines in human and animal bodies are dynamic systems, distinctly and specifically responding to various effects. The data on the ratio of individual forms of vitamin B12 in human and animal blood and tissues as well as their alterations under physiological and pathological conditions are discussed. Differentiation of individual physiologically active forms of vitamin B12 and their estimation is very important and may contribute to elucidation of molecular mechanisms of impairments in cobalamine metabolism in various diseases.