A new mass-spectrometer leak detector was investigated based on the use of an iron-shrouded axially symmetrical magnetic lens for focusing ions in the low mass range. By taking precautions to keep the hydrogen background at a low and stable level (by the use of a mercury-vapour pump, liquid-air traps and clean construction) it was possible to use hydrogen as a search gas. Leaks of 5 × 10^-10 torr litre/s corresponding to an indication of a change in ion current reading of about 3 × 10^-13 A (for hydrogen) were consistently detected with an almost instantaneous response when testing equipment of a few litres volume using either hydrogen or helium as search gas. Some measurements of the mass spectrum obtained with this instrument are given. A very reliable and convenient version of a compressed-powder leak is also described.