Abstract: A leak integrity testing apparatus (36) is provided which is adapted to be inserted within a liquid-delivery line, as for example an oil well tubing string (26), to permit in situ testing of the line or string (26) without the need for removal or inspection of the line. The apparatus (36) is a tubular body preferably including an interconnected tubular inlet (38) and outlet (40); a liquid pressure-responsive, metallic burst disc (42) is located between the inlet (38) and outlet (40). The disc (42) includes a peripheral flange (62) and an inner concavo-convex burst region (64). A score line (82, 86) is located on the concave face (66) of the burst region (64), and is of differential depth, having a reduced-depth hinge area (84, 88) serving to prevent detachment of the burst region (64) upon actuation of the disc (42).