Abstract: A system for continuously monitoring and/or controlling blood hematocrit levels is disclosed. The system includes a photometric hematocrit sensor which is coupled to a linear response meter, and in one embodiment a separate sensor is coupled to a three-way solenoid valve. The sensor is responsive to the hemoglobin density in the optical path. The valve system automatically causes diversion of solution having a degree of hematocrit dilution below a set minimum. The sensor is independent of the amount of oxy-hemoglobin (HbO.sub.2) and reduced hemoglobin (HHb) and the system allows for instantaneous detection of improper hematocrit levels and obviates the need for time-consuming and unreliable centrifugal micro-hematocrit tests.