The printed circuit board testing apparatus includes resiliently loaded probe pins carried in a support so that the pins can individually move relative to the support. A sealing means is positioned between the support and the printed circuit board, and is effective in operation to form a closed boundary enclosing a space occupied by the probes and the circuit connections and to enable a pneumatic clamp seal to be produced between the sealing means and the circuit board, to hold the probe pins in contact with the circuitry of the board.