Knowledge of the nature of exchangeable (labile) intracellular Zn(II) is increasingly important for biomedical research. The detection and quantitative determination of Zn(II) ions is usually performed by using Zn(II)-specific fluorescent sensors, among which 2-[2-[2-[2-[bis(carboxylatomethyDamino]-5-methoxyphenoxy]ethoxy]-4-(2,7-di fluoro-3-oxido-6-oxo-4a,9 a-dihydroxanthen-9-yl)anilino]acetate (FluoZin-3) has been used most widely. Selectivity of this sensor for Zn(II) over other divalent cations was demonstrated, but possible interference in its performance by other compounds has not been investigated. Many potential low molecular weight ligands for Zn(II) ions (LMWLs) are abundant in the cell. In this study we demonstrate that FluoZin-3 is susceptible to competition for Zn(II) from LMWLs and also forms strong ternary complexes with some of them. We determined the set of conditional stability constants K-c(tern) for ternary Zn(FluoZin-3)(LMINL) complexes using fluorescence titrations and applied it to model the response of exchangeable zinc to FluoZin-3. We found that competition and formation of ternary complexes with LMWLs together strongly affect (net reduce) the Zn(FluoZin-3) fluorescence. This effect may cause a significant underestimation of exchangeable Zn(II). We also demonstrated a strong pH dependence of this effect. Reduced glutathione (GSH) emerged as the most important Zn(II) partner among the LMWLs, characterized with K-tern = 2.8 +/- 0.2 X 10(6) M-1. Our experiments and calculations suggest that Zn(LMVVL) complexes contribute to the exchangeable cellular zinc pool.