Allergic asthma is associated with the recruitment of activated inflammatory cells after allergen challenge. Surface expression of CD14 has been proposed as a marker of cell activation and differentiation. We therefore measured CD14 expression on activated macrophages and granulocytes as well as soluble CD14 (sCD14) concentrations in peripheral blood and bronchoalveolar lavage (BAL) following segmental allergen provocation (SAP) with individually standardized doses of allergen in eight patients with allergic asthma. Two segments of the right lung were challenged with allergen. Two segments of the left lung, into which saline was instilled, served as controls. CD14 expression on macrophages and granulocytes was determined by flow-cytometry and concentrations of interleukins and sCD14 were analysed by enzyme-linked immunosorbent assay (ELISA) 10 min and 18 h after challenge. Soluble CD14 concentrations remained unchanged in BAL fluid after saline challenge and 10 min after SAP, but increased significantly 18 h after SAP. Although macrophage numbers increased 18h after SAP, CD14 expression on these cells did not change. Unlike macrophages, granulocyte numbers correlated with sCD14 levels 18 h after SAP while their CD14 expression decreased significantly. Furthermore, sCD14 correlated with interleukin (IL)-13 concentrations 18 h after SAP. An increase in soluble CD14 can be observed 18 h but not 10 min after segmental allergen provocation suggesting local release of this surface antigen. Our findings imply that CD14-mediated cell activation following segmental allergen provocation could play a role in asthmatic inflammation.