Abstract: Apparatus is provided for detecting ferrous contamination in a fluid such as engine transmission lubricant. The apparatus has two spaced apart electrodes, a magnetic flux extending between the electrodes the lines of force of which are substantially rectilinear, the electrodes being connectable to a circuit for signalling when an electrically conductive path is formed from one electrode to the other by metal particles attracted to the flux and formed into an electrode spanning bridge. For preference the sensor is a hollow plug, which in use is screwed into a transmission housing port, and has a flat end wall in contact with the lubricant in the housing. The plug contains a magnet having both poles disposed towards the plug end wall, a portion of the end wall acting as one electrode. A ferromagnetic disc mounted externally from the plug end wall, and electrically insulated therefrom, overlies the poles.