The periplasmic phosphate binding protein is a product of the phoS gene and is an essential component of the phosphate specific transport (PST) system, which mediates Pi uptake in Escherichia coli. The binding of Pi to periplasmic protein(s) and the kinetic parameters of Pi uptake were studied in phoT and pstB mutants of E. coli. These mutants are impaired in Pi uptake but have a periplasmic Pi-binding protein whose Pi-binding acpacity was estimated by the retention kinetics. The Pi-binding activity in two pstB mutants was found to be weaker as compared to phoT9 and the wild type. The KD values for Pi binding to periplasmic protein were determined by equilibrium dialysis. In the pstB mutants the KD value was found to be 9-31 times higher than the values obtained for the wild type and the phoT mutant. The apparent Km values for Pi uptake in one pstB mutant is 14.3 times higher than in the wild type. Vmax of the mutant is 8.3 times lower that of the wild type. The data indicate that pstB, an essential gene of the PST transport system, is promoting the binding capacity of the Pi-binding protein.