Abstract: Electro-optical apparatus for measurement of fat, protein, lactose and solids in milk wherein a milk sample in a test cell is irradiated with reference and measurement beams at differing wavelengths for fat, protein and lactose, and signals are stored indicative of uncorrected concentrations. A scaling and correction circuit includes cross-correction circuitry for compensating the effects on each reading caused by the other constituents. The signals so corrected are then provided in percentage by weight or weight over volume on suitable digital displays. For enhanced accuracy of fat concentration, infrared absorption is measured at both the carbon-hydrogen stretching wavelength (3.48 microns) and the wavelength of absorption at the ester linkages (5.72 microns), and the resulting readings are combined in a predetermined function for obtaining a reading of fat concentration.