A low‐level counting system used in absolute measurements of the C12(p,pn)C11cross section is described. In this system small activities of C11produced in polystyrene‐based plastic scintillators by irradiation with high‐energy protons are detected with high efficiency by 4&pgr; scintillation counting. Background reduction is achieved by counting in a shield consisting of 12 in. of iron and 1 in. of mercury and by the use of a ring of Geiger counters in anti‐coincidence with the scintillation counter. A background of 8 counts per min was obtained for a 1.5‐in. diam, 1‐in. thick scintillator in which the over‐all detection efficiency for C11is 90%. Determination of the counting efficiency by a positron‐annihilation coincidence method is described and the results of experiments to reduce the background still further are reported. Application of this system to monitoring fluxes of energetic protons and neutrons is discussed.