We present an investigation of the photoluminescence of CdSe/ZnS quantum dots at high light intensity and in low magnetic fields. Upon increasing the magnetic field up to 90 G, the photoluminescence intensity drops. When decreasing the magnetic field back to zero the photoluminescence drop remains present. A plausible explanation is the Zeeman splitting of defect-associated energy levels under the influence of a magnetic field. The defect-trapped electrons may then be positioned at a metastable level, thereby reducing the number of recombinations. This effect may be used to control the luminescence of quantum dots.