Therapeutic drug monitoring has been an integral part of the clinical investigations of cyclosporin G (OG37-325 CsG), an analogue of cyclosporin A (CsA). In general, analytic approaches used for monitoring of CsA have been used for the measurement of CsG. However, the validity of this approach may be questioned, as there are significant differences between the two drugs in the areas of blood distribution, pharmacokinetics, steady-state concentration of metabolites, and biases with the various immunoassays adapted for measurement of the drug. Based on the above, it is not appropriate to assume that the therapeutic drug monitoring protocols previously established for CsA are applicable to CsG. The latter drug should be treated as a new compound, with the development of independent therapeutic monitoring guidelines. Similar approaches should be used for other immunosuppressive drugs.