A prototype shadow-shield whole body counter was designed, constructed and tested as a radiation protection, medical and research tool, such as in in vivo human body composition and in occupational exposure studies. In the present study potential fields of application of a prototype whole body counter were investigated carrying out measurements in groups of adult volunteers. Among others, the naturally occurring 40 K and 214 Bi whole body radioactivity was assessed. The activity of the former was correlated with the amount of the total body potassium, a quantity related to both cell mass and lean mass in the human body. Increased 214 Bi levels found in two subjects was attributed to radium intake by ingestion. Patients were found contaminated with long lived radionuclides after administration of radiopharmaceutical of short half-life, administered for either diagnosis or treatment. Among the radiation workers monitored for internal occupational exposure, two were found contaminated with small amounts of 99m Tc.