Present day steelmaking slags are being successfully used as a high quality mineral aggregate for the building industry. With this, it is of vital importance to be familiar with the technical significance of the secondary application of steel slag, because some steel slag might contain increased concentration of substances harmful to human health. In terms of steel slag impact on the environment, radionuclides are the least researched of all pollutants emitted from the metallurgical processes. This paper presents the results of radiochemical testing of steel slag and steel slag aggregates for the purpose of its use in the production of construction material. Obtained results of measurements show that 40K, 226Ra and 232Th in all examined steel slag samples have the activity concentration from 45.3 to 62.9Bqkg-1, 15.2 to 21.4Bqkg-1 and 12.9 to 15.4Bqkg-1, respectively. Results of measurements of radionuclide activity concentrations of 226Ra, 232Th and 40K in slag aggregates show similar values for all radionuclides ranges as follows: 40K from 14.1 to 23.3Bqkg-1; 232Th from 8.6 to 14.4Bqkg-1 and 226Ra from 14.8 to 26.8Bqkg-1. Activities index (I1, I2, I3) of 226Ra, 232Th and 40K were compared with values recommended by Croatian legislation. Radium equivalent concentrations (Raeq) of 226Ra, 232Th and 40K for examined steel slag and steel slag aggregates are harmounious with the results presented by other authors for the same by-product. The testing has been conducted on steel slag created during the production of carbon steel by electric arc furnace in Steel Mill of CMC Sisak, Croatia.