Antiplatelet serum (APS) has been used to investigate the role of platelets in lung injury. However, the effect of APS on pulmonary intravascular macrophages (PIMs) is unknown. Therefore we investigated the effects of APS on PIMs. A bolus injection of APS raised pulmonary arterial pressure with the production of thromboxane. Fluorescence- and electronmicroscopy revealed that PIMs engulfed APS-platelets complexes after the APS injection. Repeated injections of APS increased the ratio of PIMs which engulfed platelets and the number of platelets in a PIM, but attenuated phagocytosis of liposomes by PIMs to decrease liposome-induced pulmonary hypertension and production of thromboxane. From the above data it was concluded that APS injection causes PIMs to engulf APS-platelets complexes, and that APS injection attenuates the phagocytosis of liposomes by PIMs, and thereby diminishes liposome-induced pulmonary hypertension and production of thromboxane.