Prostaglandin E1 (PGE1) is frequently used as a stabilizing agent while isolating platelets. The platelet-stabilizing effect of PGE1 is mediate through the PGE1 receptor and the consequent increase of intracellular cAMP. The discoid shape of platelet is thus maintained. Recently, we have studied the effect of PGE1 on platelet deformability with micropipette aspiration technics, using pipettes with radii (Rp) of 0.26-0.36 micron. Platelets, obtained from platelet-rich plasma, and suspended in Hank's buffer containing BSA, glucose, adenosine and theophylline, were subjected for micropipette aspiration test within 2 hr after sampling. In general, PGE1 treated platelets (greater than 75%) remained as non-activated form as revealed under video microscopy and maintained their discoid shape for as long as 6 hr. The time course of the extension length (Dp, in micron) of the platelets in response to aspiration with a negative pressure of 5 cm H2O (tension = 0.15 dynes/cm) was analyzed by using a digital image analyzer system (IRIS 3120 with IP-512, with a resolution of 0.076 microns). A computer program based on the algorithm of real-time reading of image intensity was developed to facilitate real-time data acquisition. The data of Dp/Rp vs time (sec) were fitted by using a second-order equation. The deformability of PGE1 (0.1 microM) treated platelet was decreased significantly as compared with control. These results indicate that the platelet stabilization by PGE1 is accompanied by a decrease in platelet deformability.