A flow cytometry system for sorting a mixture of particles including particles having a characteristic A and particles having a characteristic B, the system including: a fluid delivery system for delivering a fluid containing the particles; a flow cytometry apparatus for receiving the fluid, forming it into a stream and using flow cytometry to classify the particles according to the characteristics; and a fluid switching sorting system for sorting selected particles in the stream according to the classification and according to a sorting strategy to provide at least one population containing desired particles, characterised in that the system includes: a control responsive to information received from the flow cytometry apparatus for controlling the fluid switching sorting system to vary its sorting strategy or for controlling the fluid delivery system to vary the rate at which fluid is delivered as a function of at least one of the following: the purity of the said at least one population with respect to either characteristic A particles or characteristic B particles; and the quantity of characteristic A particles or characteristic B particles in the said at least one population relative to the total quantity of characteristic A particles or characteristic B particles in the stream. a fluid delivery system for delivering a fluid containing the particles; a flow cytometry apparatus for receiving the fluid, forming it into a stream and using flow cytometry to classify the particles according to the characteristics; and a fluid switching sorting system for sorting selected particles in the stream according to the classification and according to a sorting strategy to provide at least one population containing desired particles, characterised in that the system includes: a control responsive to information received from the flow cytometry apparatus for controlling the fluid switching sorting system to vary its sorting strategy or for controlling the fluid delivery system to vary the rate at which fluid is delivered as a function of at least one of the following: the purity of the said at least one population with respect to either characteristic A particles or characteristic B particles; and the quantity of characteristic A particles or characteristic B particles in the said at least one population relative to the total quantity of characteristic A particles or characteristic B particles in the stream. a control responsive to information received from the flow cytometry apparatus for controlling the fluid switching sorting system to vary its sorting strategy or for controlling the fluid delivery system to vary the rate at which fluid is delivered as a function of at least one of the following: the purity of the said at least one population with respect to either characteristic A particles or characteristic B particles; and the quantity of characteristic A particles or characteristic B particles in the said at least one population relative to the total quantity of characteristic A particles or characteristic B particles in the stream. the purity of the said at least one population with respect to either characteristic A particles or characteristic B particles; and the quantity of characteristic A particles or characteristic B particles in the said at least one population relative to the total quantity of characteristic A particles or characteristic B particles in the stream.