The usual in vitro methods for studies of platelets after storage do not measure physiological parameters such as capability for interaction with vessel wall subendothelium during blood flow conditions. For studies reported here we adapted a technique described by Baumgartner (Microvasc Res 5:167, 1973) to quantitate platelet interactions with subendothelium during blood flow and before and after storage at 4 degrees and 22 degrees and by a modified 4 degrees procedure called temperature cycling. The results indicated that platelets kept at 22 degrees for 72 hr interacted with subendothelium in quantities which were less than 10% that of fresh controls. However, storage at 4 degrees and by temperature cycling produced interactions equal to 71% and 42% of controls, respectively. The data also indicated that utilization of Baumgartner's technigue as described here will (1) provide an in vitro technique for identification of qualitative defects responsible for the delayed hemostatic effectiveness of 22 degrees-stored platelets and (2) permit an objective and quantitative analysis of prospective improvements in storage before in vivo function testing is necessary.