Essential to the evaluation of (1) the pharmacokinetics, (2) concentration-effect relationships, and (3) the application of therapeutic drug monitoring, during new immunosuppressive drug clinical trials, is the development of validated analytical methodology for the measurement of pharmacologically active drug and metabolites in biofluids and tissues. The characteristics of analytical methodology developed for cyclosporines A and G, FK-506, mycophenolate mofetil, and rapamycin during clinical trials will be described. The advantages of establishing validated analytical methodology as early as possible during clinical trials include: (a) early identification of metabolites and their quantitative and pharmacological significance; (b) early development of interpretable PK and PK-PD data; (c) accrual of experience that will be directly useful in patient monitoring after drug approval; (d) optimization of analysis conditions; and (e) early development of reference methodology for therapeutic drug monitoring tests. A suggested set of performance criteria for drug analysis during clinical trials and thereafter will be presented.