The manganese concentration in the Arundel clay formation, Prince Georges County, Maryland, was determined from a borehole by using delayed neutron activation. The neutrons were produced by a 100 mu g 252 Cf source. The 847 keV gamma ray of manganese 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 technique measured the concentration ratio of manganese to aluminum. This ratio, when combined with an estimate of the aluminum concentration of the clay, made it possible to determine the percentage concentration of manganese without using a test-pit calibration facility. The measurements were made by using an NaI(Tl) scintillation detector and a Ge(HP) solid-state detector cooled by solid propane. A two-pass technique had to be used with the scintillation detector because Compton background from the 1 779 keV photopeak of aluminum masked the manganese line. The Compton background did not interfere when the solid-state detector was used. The borehole measurements compared favorably with a chemical core analysis and were unaffected by water in the borehole.