Mixtures of powders of gallium nitride and manganese were annealed in a stream of ammonia at temperatures from the range of 1200-1250 degreesC. The procedure resulted in preparation of bulk single crystals of gallium nitride of dimensions up to 2.7 x 1.5 x 0.5 mm and containing up to 2% of Mn by weight. The influence of temperature, ammonia flow rate and manganese concentration in the substrate material on the doping level of the obtained bulk crystals was studied. The concentration of manganese was determined by means of an electron microprobe method. Raman investigations supported the conclusion on the well ordered structure of the prepared crystals. The measurements done by a superconducting quantum interferometer showed that the crystals doped with manganese ions manifested paramagnetic properties.