In an attempt to determine possible cellular sources of thromboxane synthesis and release during intravascular complement activation, we prepared pure, viable fractions of neutrophils and lymphocytes from sheep blood. Lymphocytes were also isolated from thoracic duct lymph. Blood neutrophils and lymphocytes release thromboxane and the cells aggregate when challenged with zymosan-activated plasma. Blood lymphocytes release thromboxane in greater quantity than do neutrophils with the complement stimulus. Lymph lymphocytes, when stimulated with zymosan-activated plasma, did not release thromboxane or aggregate. These data suggest that both blood neutrophils and lymphocytes are likely sources of complement-initiated thromboxane synthesis and release in sheep.