The Kinetic properties of plasma placental alkaline phosphatase patients with Chagas' disease were studied. When Cl2 Mg was used as activator the same increase of activity (17-20 per cent) was found in the chagasic and non chagasic groups. The enzyme was not inhibited by F-ion in any of the groups. No significant differences were detected between the two groups (chagasic and non chagasic) when the enzyme was treated with inhibitors such as EDTA and L-phenylamine. However, when the CN- ion was used, the enzyme of the normal pregnant women followed a Michaelian curve, whereas in the chagasic group a sigmoideal plot was observed. Thus, the Hill coefficient was 1.1 for the normal group and over 1.5 for the chagasic.