Research carried out at IIMT Cluj-Napoca in the field of radioisotope abundances had in view the improvement and application of the methods of low-level counting to various problems in geology, hydrology, radioecology, archaeology, etc. During the last 25 years some progress has been made in improving techniques of low-level radioactivity measurements by constructing low background detection systems and by developing new and rapid methods for detector calibration and sample preparation. So, the conventional methods for U, Ra, Th and Rn determination in natural waters were completed with the construction and development of gas-filled proportional counters for tritium and carbon-14 analyses in the environment. According to a national program of radioecological studies, analyses of elemental U, Ra, Th and Rn in waters from the Danube Delta area were begun in 1991. The artificial component of radioactivity was also followed in fallout, rainfall, soil, ground water, food, drugs and pharmaceutical plants especially after the Chernobyl disaster.