We describe a flow cytometric method to evaluate upregulation of peripheral blood neutrophil and monocyte integrin CD11b in vivo. To avoid spontaneous upregulation in vitro, buffy coat cells were separated on ice and all subsequent cell handling steps were carried out at 0-4 degrees C. Such leukocytes were 95-100% viable, as determined by PI staining. Buffy coat leukocytes were double-stained with CD11b PE-conjugated and CD14 FITC-conjugated monoclonal antibodies and, in addition, with the nucleic acid dye LDS-751. After staining, firstly, LDS-751 positive (+ve) leukocytes, and, secondly, CD14 +ve monocytes were collected in live mode. Aggregated and irrelevant cells were gated out on the basis of their LDS-751 staining pattern and cellular light scattering properties, and the CD11b expression on neutrophils and monocytes was determined. Upregulation of CD11b in vitro was significantly affected by factors such as cell handling temperature, pre-fixation of blood samples, and density gradient separation of the cells. Incubation of aliquots of buffy coat cell suspension supplemented with FMLP for 5 min or without FMLP supplement for 15 min at 37 degrees C significantly increased CD11b expression without affecting cell viability. We have demonstrated that CD11b is expressed at maximal levels on arthritic synovial fluid neutrophils and CD14 +ve cells, and at increased but submaximal levels on peripheral blood neutrophils and monocytes of patients recovering from sepsis. The results suggest that the method can be used to evaluate in vivo upregulation of CD11b.