The degree of extracorporeal platelet sequestration in 32 patients undergoing cardiopulmonary bypass has been assessed using 111In labelled platelets and both a shadow shield whole body monitor and a standard gamma camera. The effects of ZK 36374, a prostacyclin analogue, on deposition of platelets in the oxygenator and atrial line filter were also investigated. Total platelet deposition in the placebo group ranged from 2.2% to 31.7%, mean 13.9 +/- 7.8%; n = 15, and was significantly higher than the deposition in the treatment group, range 0.7% to 9.4%, mean 3.9 +/- 2.6%; n = 16, P less than 0.001. There was also a highly significant correlation between the gamma camera and whole body monitor measurements, r = 0.99, P less than 0.001, with no systematic difference between the techniques. This study demonstrates that accurate quantitative assessment of platelet deposition can be achieved with either the gamma camera or whole body monitor. In addition, significant reduction in platelet sequestration in the extracorporeal circuit can be achieved by using ZK 36374 during the bypass operation.