Blood anticoagulated with citrate-phosphate-dextrose (CPD) was run through blood administration sets before and after Ringer-lactate solution (BP), Hidroliet, Maintelyte, Plasmalyte B and 0,9% saline. Clots, some of which were proven by electron microscopy to contain fibrin, formed distal to the filter when Ringer-lactate and Hidroliet were used, but not with other solutions. It is suggested that their formation was due to the calcium in Ringer-lactate and Hidroliet. As these two solutions are frequently used in combination with blood, especially in shocked patients, and as the clots would presumably be filtered out in the lung, it would seem wise to use either calcium-free Ringer-lactate solution or some other crystalloid solution such as 0,9% saline or Plasmalyte B.