Several variables which influenced the cellular translocation rate of IT have been analyzed in different systems in an effort to improve the efficacy of ITs. Although some conflicting data were reported, several parameters for improving the efficacy of ITs were found: 1) IT was internalized faster when more target antigens were occupied by IT, implying that a highly expressed antigen would be a better target for IT treatment; 2) the nature of the target antigen had an important role in the translocation of IT, presumably directing the IT to different intracellular compartments; 3) small conjugates internalized faster than large conjugates, but the slow translocation of large conjugate was compensated for by adding more toxin molecules to the conjugate; and 4) the kinetics of intoxication by IT was accelerated with certain potentiators, however, in vivo use of these agents may be limited.