Differences in the physical and chemical properties of synovial fluid from healthy and arthritic joints are detected by infrared spectroscopy. A beam of infrared light is directed at a sample of synovial fluid (either in its native form or prepared as a film) and changes in the physical and chemical properties of the fluid being analyzed are detected at one or more wavelengths to determine whether changes in the position, width, absolute intensity, relative intensity or shape of the infrared absorption have occurred which are characteristic of the arthritic condition.