A new ultra-low-background proportional counter was recently developed with an internal volume of 100 cm3 and has been characterized at pressures from 1–10 atm with P-10 (90% Ar, 10% methane) gas. This design, along with a counting system providing event digitization and passive and active shielding, has been developed to complement a new shallow underground laboratory (30 m water-equivalent). Backgrounds and low-level reference materials have been measured, and system sensitivity for 37Ar has been calculated.