A commercially available radionuclide calibrator of the type Curiementor 3 (PTW, Freiburg, Germany) was calibrated using activity standard solutions, from the Physikalisch-Technische Bundesanstalt (PTB), of a number of photon-emitting radionuclides, in particular those used in nuclear medicine: (18)F, (51)Cr, (67)Ga, (99m)Tc, (111)In, (123)I, (125)I, (131)I, (186)Re, (201)Tl, and for beta-emitting nuclides: (32)P, (89)Sr, (90)Y, (204)Tl. An energy-dependent efficiency curve was established to calculate efficiencies for photon-emitting radionuclides. The differences between experimental and calculated radionuclide efficiencies of the measuring system are of the order of a few percent. The performance of the system was checked for instrument stability, using a long-lived (137)Cs source, and for linearity using the decay of a (99m)Tc source. The calibration measurements were carried out with 2g of solution in standard PTB-type glass ampoules. The dependence of efficiency with varying solution mass and changing ampoule geometry is studied for some of the radionuclides. For example, geometry correction factors are determined for various solution volumes or ampoules with different wall thickness. The measuring system will enable the National Centre of Metrology (NCM) of Bulgaria to provide calibration services for activity measurements, which are traceable to PTB standards, and to organize national comparisons in order to improve the quality of measurements for nuclear medicine in Bulgarian hospitals. Furthermore, it should enable NCM to declare Calibration and Measurement Capabilities (CMCs) and to submit radioactive solutions to the International Reference System (SIR) at the BIPM in order to compare the results with Key Comparison Reference Values (KCRVs).