The chemical properties of skin angiotensin-converting enzyme were characterized in mouse and human. In newborn mice, dermis contained almost all activity (1.3 mU/mg), of which 30% was in the 22,000 kappa g supernatant and 70% in the pellet which could be solubilized by Triton X-100. The activity increased sharply during the first 6 weeks after the birth. By gel filtration, the enzyme in the supernatant was Mr 330,000, but the solubilized enzyme was Mr 430,000 in the presence of Triton X-100. By electrophoresis, the 2 enzyme fractions demonstrated a charge difference. The enzymes showed the same pH optima of around 8.1 and 7.7, and Km values of 2.6 and 0.11 mm for Hip-His-Leu and angiotensin I, respectively. Sensitivity to known inhibitors and heat stability of the enzymes in 2 fractions were similar. In the human, 56% of the activity in skin homogenates (1.5 mU/mg) appeared in this supernatant and 44% in the pellet. Both soluble and solubilized preparations showed enzyme activity with 2 different molecular weights, 330,000 and 430,000. The human enzymes had chemical properties similar to the mouse enzymes, but their affinity for Hip-His-Leu and angiotensin I were 1.6 and 2 times higher than those of the mouse enzymes.