In the application, the change in the magnetic state of the haemoglobin caused by the malarial infection is exploited by detecting suitable properties of haemozoin which are dependent on the application of a magnetic field. FIG. 1 shows apparatus, shown generally at (10), for performing magneto-optical detection using photo-acoustic techniques. The apparatus (10) comprises a light source (12), producing a beam of optical radiation (14) which passes through a polariser (16), a variable LC retarder (0 or 180.degree. retardance) (18), and a (chopper 20), before impinging on a sample (22) held in a sample holder (24). The sample is in direct contact with an acoustic detector (26). The apparatus (10) further comprises an electromagnet (28), and a Gauss meter (30) can be utilised to measure the applied magnetic field strength. Advantages associated with this approach are the--possibility of making in vivo measurements, and the avoidance of problems of optical scattering associated with conventional optical measurements on turbid liquids such as whole blood.