The incorporation of ⁵⁹Fe into heme and the synthesis of δ‐aminolevulinic acid by leukocyte homogenates were studied. Whereas polymorphonudear peritoneal exudates from rabbits, as well as leukocytes from healthy adult volunteers and patients with chronic leukemia, lacked the capacity to incorporate radioiron into heme, immature leukemic cells from patients with acute leukemia possessed iron‐incorporating activity. In addition these cells could utilize δ‐aminolevulinic acid as substrate for heme synthesis. Delta‐aminolevulinic acid synthetase activity could not be demonstrated in immature leukemic cells nor could they utilize glycine as substrate for heme synthesis. This work does not reveal whether the heme‐synthesizing capacity in immature leukemic leukocytes is a function of this immaturity or of malignant transformation.