Platelet concentrates amounting to 142 X 10(9) +/- 11 X 10(9) cells are prepared by cytopheresis from one liter of blood. Ten of the concentrates are labelled with 51Cr and reinfused autologously. The other ten concentrates are frozen with a controlled rate freezer in a medium containing 10% DMSO and 5% glucose for 7 days at -196 degrees C. The fresh platelets have a 51Cr 24 hrs. recovery of 68 +/- 7% and a survival rate (T 1/2) of 10 +/- 1 days. The frozen preserved platelets have a 51Cr 24 hrs. recovery of 54 +/- 6.1%, a T 1/2 of 8.1 days, and significantly increased liver sequestration. There is a correlation between the reversal reaction in vitro after hypotonic shock and the 51Cr 24 hrs. in vivo recovery of fresh platelets and the 51Cr 1st hr. recovery of frozen platelets.