The design, construction and operation of a monitor for detecting metallic particulate contamination in lubricating oils is described. The principle of operation is that lubricating oil flows through a transducer coil, causing small changes in the inductance of a coil as any metallic particles pass. The coil is used as the resonating inductance in an oscillator, whose frequency is compared to a fixed crystal controlled oscillator in a phase locked loop (PLL). The PLL is arranged so that it compensates automatically for any slow drift in the oscillator frequency, while transient disturbances produced by metal particles can be detected and amplified. The direction of the frequency shift allows discrimination between ferrous and non-ferrous particles and the signal amplitude gives a measure of debris size and concentration.