Abstract: The method, and the apparatus for carrying out the steps of the method, are utilized in the centrifugal liquid processing of whole blood wherein whole blood is withdrawn from a donor, passed through a fluid system and the blood is centrifuged to separate the same into at least three components. One component which is to be collected is withdrawn at a fixed rate of volumetric displacement while the other two components are withdrawn at variable rates of volumetric displacement. The other two components are recombined and returned to the donor. The withdrawal of the one component is optically monitored to determine whether or not either one of the other components is being mixed with the one component. When such a mixing is sensed, the rate of withdrawal of one of the other two components is increased while the rate of withdrawal of the other components is decreased thereby to ensure withdrawal of the desired one component, e.g.