The following summarizes our observations on in vitro viability and function and ultrastructure of platelets stored at 4 degrees, 22 degrees, and by temperature cycling: 1) Continuous 4 degrees storage for 24 hours resulted in irreversible loss of discoid shape and eliminated platelet capability for microtubule reassembly. 2) Abnormal in vitro platelet function (aggregation) and significant decreases in viability (recovery from hypotonic shock) occurred concomitantly with losses in microtubule reassembly capabilities. 3) Platelets demonstrating a capability for microtubule reassembly during 4 degrees storage (temperature cycled) produced normal aggregation responses and gave enhanced viability scores. 4) Platelets stored by temperature cycling for 96 hours produced viability scores equal to those produced after storage at 4 degrees for 24 hours or those kept at 22 degrees for 72 hours.