The platelets used in this study were collected by serial centrifugation, and within four hours of collection were frozen with 5% dimethylsulfoxide (DMSO) at an overall rate of 2 to 3 C per minute by storage in a mechanical refrigerator at -80 C. The frozen platelets were stored for four to ten weeks before thawing and washing. After washing, the units were kept at room temperature for six to eight hours before transfusion. The units were pooled, and an average of eight units was given to each of four patients, with a range of three to 14 units per transfusion. In vitro recovery after washing was about 65 per cent and in vivo recovery of the 51chromium labeled (51Cr) platelets was about 35 per cent. The infusion of these previously frozen washed platelets corrected prolonged bleeding times in patients. The homologous platelets were transfused along with other blood products to treat patients with hematologic disorders. The circulation and function of the donor platelets were influenced by compatibility of the platelets, the quality of platelet preservation and the patient's disease state.