Human body models (phantoms) have been used to study relations between efficiency in measurement of radionuclide activity and measurement conditions. A comparison is made between the activity measurement with and without a motion. When the phantom is moved under the detector, an absolute efficiency increase of 30-40% in homogeneously distributed activity and 2-3 times increase in activity concentrated in the lungs are found. Incorrect calibration is found to introduce an error of 30-40% in the measurement with motion while without motion the error can reach 150-300%. The effects of object weight, size and shield are studied. A procedure for accurate measurement has been developed. The incorporated activity in a human body has been measured using this procedure. Two Kozloduy-2 staff members have been subject to measurement. Cs-137 activity was homogeneously distributed with values 175 Bq in person X and 1050 Bq in person Y. Co-60 activity was concentrated in the lungs with values of 110 Bq and 470 Bq respectively. 14 refs., 5 tabs.