The activity of 40K in natural potassium as derived from the recent literature varies between 27.33 to 31.31 Bq g-1 of potassium. This paper reports measurements by gamma-ray spectrometry and compares these with independent calculations, yielding values of 30.18 +/- 0.72 Bq g-1 and 31.00 +/- 0.33 Bq g-1 respectively.