Metamorphosis was induced in neotenic axolotls by immersion of the animals in a solution of thyroid hormone. Hematology of the axolotls was examined before, during, and after metamorphosis. There was a transient decrease in numbers of certain white blood cells during metamorphic climax and a permanent shift in the pattern of circulating cells. The eosinophilic granulocyte was the dominating leukocyte type in neotenes and in metamorphosing animals up to midclimax. Lymphocytes and neutrophilic granulocytes (polymorphs) significantly decreased during midclimax. In postmetamorphic axolotls, lymphocytes and polymorphs predominated. The observed decrease of some leukocytes in metamorphosing animals accords with a transient immunosuppression at metamorphic climax. Metamorphosed axolotls showed a humoral immune response (increase in circulating plasma cells) after repeated antigen challenge, whereas neotenic axolotls did not. Alterations in both cellular and humoral immunity are suggested to occur in both young and adult axolotls following experimental induction of metamorphosis.