The biological compact disc (BioCD) is a sensitive detection platform that detects immobilized biomolecules on the surface of a spinning disc by quadrature laser interferometry. Spinning-disc interferometry (SDI) has the advantage of operating faraway from the 1/f system noise which has a 40 dB per octave slope, thus reducing the detection noise floor by more than 50 dB compared to static interferometric detection techniques. Three quadrature classes of BioCD have been previously reported: micro-diffraction, adaptive optical and phase contrast. In this paper, we introduce a new class of BioCD, the in-line quadrature class, which has achieved a new level of simplicity and sensitivity. A silicon wafer coated by a layer of SiO2 is used as a substrate for immobilized biomolecules. The thickness of the SiO2 layer is chosen so that light reflected from the SiO2 surface on top and the silicon surface below is approximately in phase quadrature. Protein molecules scatter the incident light, adding a phase shift linearly proportional to the mass density of the immobilized protein, which is converted to a far-field intensity shift by quadrature interference. Patterning of protein is achieved by spot printing with a jet printer, which produces protein spots 0.1 mm in diameter. We demonstrate the sensitivity of the in-line quadrature BioCD by an equilibrium dose response experiment on a disc printed with 25,000 proteins spots with a detection limit of 1 ng/mL when divided into 32 virtual wells and treated as 32 separate assays. This current performance is not a fundamental limit, and improvements in disc uniformity will enable scaling up to large numbers of individual assays per disc.