The fluorimetric determination of tetracycline is usually carried out in the presence of some metals that, through the formation of a complex with this antibiotic, enhance its fluorescence emission, giving more sensitive determination methods. It is well established that magnesium is one of these metals. However, it is possible that higher signals do not mean a real improvement in the quality of the analytical method. In this work, the univariate and multivariate fluorescence determination of tetracycline is performed in the presence and absence of Mg2+, comparing the quality of the analyses through some performance characteristics that, according to Commission Decision 2002/657/EC define the functional qualities of analytical methods. The methods with the best performance characteristics were multivariate determinations carried out in the absence of Mg2+, both when emission or excitation spectra were taken, the decision limits (CCα) being 13.1 and 20.1 µg/L and the detection capabilities (CCβ) 25.3 and 38.5 µg/L, respectively. This study points out through a case study that higher analytical signals do not necessarily mean better performance characteristics of a method of analysis. Copyright © 2007 John Wiley & Sons, Ltd.