Human blood platelets in ACD plasma were stored in sterile plastic bags for 24-96 h at the ambient temperature without agitation. No spontaneous aggregation nor bacterial contamination were noted. A progressive loss of the following parameters was seen: platelet count; ADP-, thrombin-, collagen-, and epinephrine-induced platelet aggregation; platelet factor 3 activity; reversible response to the osmotic shock; volumetric constants; amount of UV-absorbant material; 14C-5-hydroxytryptamine and 3H-adenosine uptake and release; platelet population pattern and glycogen synthesis activity. The platelet aggregation and release, the osmotic shock test, and the platelet population pattern appear to better illustrate the early changes during platelet storage and to account for the 25-42% of recirculation of 24 h stored platelets administered into thrombocytopenic patients. As stated by Murphy and Gaardner, platelets stored at 20-22 degrees C with or without agitation, although having failed to retain total functional and biochemical capacities, paradoxically seem to recuperate in vivo as shown by survival data and hemostatic effects.