A Luminometer is a medical instrument that performs assays by measuring the light produced by the bioluminescent reaction between a biological sample and other reagents. Because of the extremely low intensity of the light, the measurement is normally done by a photomultiplier tube (PMT) operating in the photon counting mode. This paper describes the design and development of such a system as a part of a new Luminometer instrument. This Luminometer utilizes a microprocessor controller in the instrument as well as an "AT class" PC used as the operator interface. This paper describes the design procedures, the testing carried out on the prototype instrument, and some design changes planned for the next generation instruments.