AbstractEosinophils possess membrane receptors for the Fc portions of IgG and IgE and also for fragments of the complement component C3, i.e. C3b (CR1) and C3bi (CR3). A number of mediators of inflammation are known to be able to upregulate eosinophil functionin vitro, including increased expression of membrane receptors.In this study we have utilised a rosette technique to demonstrate the ability of the autacoids PAF‐acether, its inactive precursor lyso‐PAF, leukotriene B4and histamine to enhance eosinophil immunoglobulin receptor expression. Human monoclonal immunoglobulins co‐valently linked to sheep erythrocytes (E) were used as a measure of functional IgGl and IgE receptor expression, while C3b coated E were utilised as a measure of complement receptor expression. Both PAF‐acether, leukotriene B4and to a lesser extent, histamine gave significant increases in eosinophil IgGl and IgE rosette numbers in a time and dose dependent manner. Lyso‐PAF had no such effect. Pre‐incubation of eosinophils with PAF‐acether gave dose‐dependent significant increases in C3b rosetting.These results show that eosinophils can be activatedin vitroby a number of autacoids as measured by functional increases in membrane receptor expression. This may represent a facet ofin vivoeosinophil activation with possible implications for amplification of the inflam