The Mn concentration in the Arundel clay formation, Prince Georges County, Maryland, was determined from a borehole by using delayed neutron activation. Then neutrons were produced by a 100 mu g 252Cf source. The 847 keV gamma ray of Mn was detected continuously, and its counting rate was measured at intervals of 15 s as the measuring sonde was moved at a rate of 0.5 cm/s. The borehole measurements compared favourably with a chemical core analysis and were unaffected by water in the borehole.-from Authors