Total potassium of ten human subjects has been determined by gamma spectrometry following the method Of MILLER and MARINELLI. Preliminary in vitro trials have given an estimation of the different factors influencing the accuracy of the result. With K42 standardization, the results are reproducible with an error of 3.2 per cent. But this standardization introduces conditions which make clinical applications difficult. We propose a detector with a nearly 4 pi geometry, formed of a hollow plastifluor cylinder with the photomultipliers located at the end in a crown arrangement.