The purpose of this study was to determine the molecular weights of the mature subunit of embryonic chick liver delta-aminolevulinate synthase and of its putative precursor fom. Although an active enzyme with a subunit molecular weight of 51,000 could be purified from the livers of porphyric embryos, it was determined by immunoreplicate electrophoresis analyses of sodium dodecyl sulfate-solubilized liver homogenates and mitochondria from porphyric embryos that the actual molecular weight of the enzyme's subunit was 65,000 +/- 2,000. These results suggested that the usual procedure for purifying delta-aminolevulinate synthase from chick embryo yielded a partially degraded enzyme. When the products of cell-free translations of mRNA extracted from livers of porphyric embryos were analyzed, they contained a polypeptide of 75,000 +/- 400 daltons which specifically cross-reacted with anti-delta-aminolevulinate synthase. It is concluded that subunit of embryonic chick liver delta-aminolevulinate synthase present in mitochondria has a molecular weight of 65,000 and appears to be synthesized as a precursor of 75,000 daltons.