Tobramycin usage, as for most aminoglycoside antibiotic usage, may require careful monitoring to avoid irreversible toxicity. Several methods for tobramycin quantitation are available. The purpose of this study was to evaluate the performance of solid-phase immunofluorescence as an alternative. Tobramycin was quantitated in the sera of 81 patients by radioimmunoassay and solid-phase immunofluorescence. No statistically significant or medically important difference was demonstrated (bias, 0.006 μ/ml,t=0.09,s=1.02,r= 0.97). Within-run and run-to-run precision for the two methods were comparable. Interference by gentamicin could not be demonstrated. Solid-phase immunofluorescence may be an acceptable alternative method for tobramycin quantitation in some laboratories.