Abstract: A liquid flow regulating device which is particularly useful in the process of plasma pheresis. The regulating device includes a hollow member communicating with a donor through a tube passing through one end of the hollow member. There is a liquid flow control member having a passageway therethrough closing the other end of the hollow member. A dial member rotatably mounted with respect to the flow control member. At least two passageways through said dial member adapted to be aligned with said passageway in the control member. Selection means on said dial member and said control member for positively positioning said control member passageway with one of said dial member passageways to control the liquid flow.