The luminescence quantum efficiency of nanocrystalline ZnS:Mn2+ is studied to provide a better understanding on how the quantum efficiency is influenced by the Mn2+ concentration, the nature of the passivating polymer, and the synthesis conditions. The results show an increase of the luminescence quantum efficiency with the Mn2+ concentration in the nanocrystals for very low Mn2+ concentrations. Between 0.3 and 1.5 at. % Mn2+ the increase in quantum efficiency levels off, to reach an almost constant level between 1.5 and 5.6 at. % Mn2+. Up to a concentration of 5.6 at. %, no concentration quenching is observed. The influence of the nature of the passivating polymer is investigated by comparing the luminescence quantum efficiencies for nanoparticles coated with poly(vinylbutyral) (PVB), poly(vinyl alcohol) (PVA), methacrylic acid (MA), or sodium polyphosphate (PP) or without a passivating polymer. For the presently used synthesis method (in water), the highest quantum efficiencies (around 4%) are obtained for nanocrystalline ZnS:Mn2+ capped with PP. Nanoparticles synthesized in a nitrogen atmosphere have higher quantum yields than nanoparticles made in ambient air. In general, large variations in luminescence properties are observed due to unintentional variations in the synthesis conditions. For research on the luminescence properties and quantum efficiencies of nanocrystalline ZnS:Mn2+, it is very important to check the reproducibility of results, to standardize synthesis conditions, and to measure absolute quantum efficiencies rather than relative changes in luminescence intensity.