A comparative study was performed on platelets frozen with dimethyl-sulfoxide (D.M.S.O.), glycérol and dimethyl-acetamide (D.M.A.C.). The results of tests realized in vitro (platelet yields after thawing platelet aggregation, hypotonic stress, release of enzymes and ultrastructure), and in vivo (life span and recoveries after transfusion) have clearly shown that D.M.S.O. is the best cryoprotective agent. The results obtained with glycerol were less interesting and more clinical data are needed in order to study the hemostatic effectiveness of platelets preserved with this agent. Dimethyl-acetamide gave the worse results and in our opinion, must not be used for the cryopreservation of platelets.