Radioimmunoassay and enzyme immunoassay methods for analysis of serum gentamicin levels have been shown to be comparable. The purpose of this study was to determine if serum concentration-time data from the same patient assayed by radioimmunoassay and enzyme immunoassay would provide the same estimates for half-life, elimination rate constant, distribution volume, drug clearance, and gentamicin dose. A total of 103 pre- and postinfusion serum samples were obtained from 32 patients. The samples were divided and assayed by radioimmunoassay and enzyme immunoassay. Serum concentration-time data were fitted to a one-compartment model, and kinetic calculations were performed using the method of Sawchuk et al. (Clin. Pharmacol. Ther. 21:362-369, 1977). While good correlation was established between the two assay methods, significant (P < 0.05) mean differences were seen in distribution volume (25%), gentamicin clearance (15%), and half-life (11%), using the quantitative data from both methods. Because of differences noted in these pharmacokinetic parameters, significant differences were also noted in dosage calculations. The authors conclude that there are differences in the pharmacokinetic parameters obtained using results from the radioimmunoassay and enzyme immunoassay. These differences also translate into significant differences between dosage recommendations when individualization of the gentamicin regimen is attempted.