Epidemiological studies have suggested that subjects who develop ischaemic events have an elevation in their numbers of white blood cells. White cells are similar to platelets in that they can adhere, aggregate and undergo a release reaction. We wished to investigate white cell aggregation in vascular diseases, and have developed a new technique to measure whole blood white cell aggregation. This paper describes the method with special reference to the aggregating agents used. The effect of Iloprost, a synthetic prostacyclin analogue, on white cell aggregation has also been investigated.