Cytochemical properties of atypical erythroblasts and 'histioid' cells from the bone marrows of two patients with acute erythremic myelosis (Di Guglielmo's disease) were studied. In addition to strong PAS and strong nonspecific esterase positivity, intense specific esterase positivity was also detected in the majority of the erythroid precursors. Since specific esterase activity is considered to be a feature of granulocytic cells, its presence in erythroid precursors raises the possibility that the atypical erythroblasts of acute erythremic myelosis may share close metabolic relationships with granulocytic cells.