Some properties and kinetics of the free and bound serine acetyltransferases (SATs) andO-acetylserine sulfhydrylase-As (OASS-As) fromEscherichia coliwere investigated. In some cases, SATΔC20, deleting 20 amino acid residues from theC-terminus of the wild-type SAT (Biosci. Biotechnol. Biochem., 63, 168-179 (1999)) was tested for comparison. The optimum pH and stability against some reagents for the free and bound wild-type SATs were similar except for the resistance to cold inactivation. The kinetics for the wild-type SAT and SATΔC20 followed a Ping-Pong Bi Bi mechanism with a mixed-type inhibition byL-cysteine. The kinetics and kinetic constants for the wild-type SAT were not changed by the complex formation with OASS-A. The optimum pH for OASS-A was shifted towards an alkaline pH by the complex formation. Thermal stability and stability against some reagents for the free and bound OASS-As were almost the same. On the other hand, the maximum velocity for OASS-A was lowered and dissociation constants for the substrates and products were increased by forming the complex with the wild-type SAT, although the kinetics for the free and bound enzymes followed the same Ping-Pong Bi Bi mechanism. From comparisons of computed courses ofL-cysteine formation fromL-serine using SAT (wild-type SAT and SATΔC20) and OASS-A with the experimental results and changes in the stability of the wild-type SAT by the complex formation, we discuss the role and significance of a complex formation for the cysteine synthetase.