Poisoning of rabbits with phenylhydrazine resulted in development of haemolytic hyperchromic anemia accompanied by impairment of hemopoiesis in bone marrow as well as by an increase of total vitamin B12 content in blood. The ration of individual forms of cobalamins was firstly estimated in blood serum of healthy rabbits and of the animals treated with phenylhydrazine. Distinct decrease in the methyl cobalamin content was observed in blood serum during spontaneous recovery. Administration of methyl cobalamin led to complete normalization of some blood and hematopoiesis patterns, as well as to restoration of total cobalamins content and the spectrum of their individual forms. Adenosyl cobalamin exhibited distinctly lower effect on the patterns studied. The data obtained suggest that methyl cobalamin possessed a lot of advantages in treatment of hemolytic anemias.