The present paper reports characteristics and specificity of the inhibitory action of $N^{alpha}-tosyl-L-lysine-chloromethyl;ketone$ (TLCK) and $N^{alpha}-tosyl-L-phenylalanine-chloromethyl;ketone$ (TPCK) on the glucose6-phosphate transporter of rat liver microsomes. The TLCK-induced inhibition was pH dependent. The inhibition constants for TPCK were determined by following pseudo-Lst order reaction mechanism. The inhibition was protected by preincubation with excess amount of glucose-6-phosphate. The results proved that (a) TLCK inactivates the microsomal glucose-6-phosphate transporter, (b) the inhibition results from the modification of sulfhydryl groups of the transporter.