A case of leukemic mastocytoma terminating in metamorphosis is reported. Three types of mastocytoma cells appeared in the peripheral blood and bone marrow, each differing in cytochemical staining and maturation ability in both in vitro and in vivo cultures. Most immature mastocytoma cells, which contained no granules and were lymphoblast-like cells, differentiated to have basophilic and metachromatically positive granules in the cytoplasm after culture in the diffusion chamber, resembling more mature cells (type II). Mature cells (type II), which had diffusely dispersed and metachromatically positive granules in the cytoplasm, came to have vacuoles with basophilic substances after culture in vitro, resembling most mature cells (type I) which contained large and clumped granules with metachromasia. These findings together with clincial findings and results of morphological studies suggest that most immature cells originate from second mature or most mature cells and effect a blastic transformation.