Fluid testing apparatus comprising: a measurement chamber having a binding site to which can attach magnetic particles which have become associated with a target substance in the fluid, the binding site comprising a plate having a coating by which magnetic particles carrying the target substance are captured; an oscillator circuit comprising inductance and capacitance in resonant combination to set the frequency of the circuit's output signal and adjustment means for altering the capacitance, wherein the inductance is influenced by the quantity of magnetic particles at the binding site and the adjustment means is arranged to alter the capacitance to maintain said frequency in the face of changes in said quantity; characterised in that the fluid testing apparatus further comprises: two permanent magnets (40, 46), each of which is moveable to vary the distance between the respective magnet (40, 46) and the measurement chamber; and a control unit which governs the position of the magnets (40, 46) such that a magnetic field within the measurement chamber can be adjusted: to a capture state, in which the magnetic particles are distributed in the measurement chamber; and by moving one magnet to its maximum distance from the measurement chamber and the other magnet to its minimum distance from the measurement chamber, to a collection state, in which the magnetic particles are urged towards the binding site. a measurement chamber having a binding site to which can attach magnetic particles which have become associated with a target substance in the fluid, the binding site comprising a plate having a coating by which magnetic particles carrying the target substance are captured; an oscillator circuit comprising inductance and capacitance in resonant combination to set the frequency of the circuit's output signal and adjustment means for altering the capacitance, wherein the inductance is influenced by the quantity of magnetic particles at the binding site and the adjustment means is arranged to alter the capacitance to maintain said frequency in the face of changes in said quantity; characterised in that the fluid testing apparatus further comprises: two permanent magnets (40, 46), each of which is moveable to vary the distance between the respective magnet (40, 46) and the measurement chamber; and a control unit which governs the position of the magnets (40, 46) such that a magnetic field within the measurement chamber can be adjusted: to a capture state, in which the magnetic particles are distributed in the measurement chamber; and by moving one magnet to its maximum distance from the measurement chamber and the other magnet to its minimum distance from the measurement chamber, to a collection state, in which the magnetic particles are urged towards the binding site. two permanent magnets (40, 46), each of which is moveable to vary the distance between the respective magnet (40, 46) and the measurement chamber; and a control unit which governs the position of the magnets (40, 46) such that a magnetic field within the measurement chamber can be adjusted: to a capture state, in which the magnetic particles are distributed in the measurement chamber; and by moving one magnet to its maximum distance from the measurement chamber and the other magnet to its minimum distance from the measurement chamber, to a collection state, in which the magnetic particles are urged towards the binding site. to a capture state, in which the magnetic particles are distributed in the measurement chamber; and by moving one magnet to its maximum distance from the measurement chamber and the other magnet to its minimum distance from the measurement chamber, to a collection state, in which the magnetic particles are urged towards the binding site.