The effect of platelet anti-aggregate agent RA-233 and prostaglandin upon PMN response to mechanical trauma was studied using the Rice University ROM-8 viscometer. Suspensions of PMN leukocytes in phosphatebuffered saline (PBS) were sheared at 1 and 3 dynes per square centimeter for ten minutes in a 37°C environment. The leukocyte suspensions were tested for alterations in total leukocyte count, changes in morphology, and variances in the PMN differential count. PMN leukocyte function was tested using several adapted assays including adhesion to glass slides, phagocytosis, and release or loss of enzymes marking the various granule populations present in PMN cell cytoplasm. Leukocytes which received 1 x 1^-6M RA-233 and 3 x 1^-6M PGE1 ten minutes prior to shearing, showed significant preservation of enzyme activity marking specific granules and reduced release of enzyme activities marking both specific and azurophilic granules, while control suspensions showed losses of enzyme activity agreeing with the results of earlier work by Dewitz (4). PMN phagocytosis, as measured by a chemiluminescence assay was significantly decreased by shear stresses of 1 and 3 dynes/cm, and was not preserved by incubation with the anti-platelet agents. PMN leukocyte adhesion to serum-coated glass slides after shear stress exposure was increased while PMN leukocytes incubated with RA-233 and PGE1 showed reduced overall adhesion in the controls, but similar increases with shear stress exposure. PMN leukocyte suspensions showed a sharp drop in electronic particle count, (EPC), following shear stress exposure, due to the combined effects of aggregation and lysis. The latter effect was apparently reduced at the higher shear stress levels by incubation with the anti-platelet agents. These results and the observations of several other groups, reviewed in the first section, lead us to propose a possible leukocyte preservational effect for the two platelet antiaggregating agents tested. The significance of shear stress trauma is reconfirmed once again by observations made in this series of experiments. The possibility that mechanical trauma may be a mediator of leukocyte dysfunction during extracorporeal circulation is discussed in the light of this new experimentation.