Neutrophil research relies largely on studies with highly purified cells. Yet the isolation procedures induce changes in surface expression of several proteins. We used a large panel of monoclonal antibodies (MoAbs) to characterize in detail the phenotypic changes during isolation and stimulation of human neutrophils. Centrifugation on density gradients appears to be the crucial step that causes an increase in expression of antigens not detectable on neutrophils in whole blood samples (cytochrome b558 recognized by MoAb 7D5; and CD10) or expressed at significantly lower levels (CD11a, CD11b, CD11c, CD13, CD16, CD45, and CD67). Other antigens were unaffected by the density gradient centrifugation step (CD32, CD54, CD58, Leu-8, HLA class I). Upregulation of antigens was also determined by stimulation of purified neutrophils. Upregulation of CD63 was an excellent marker for release from azurophil granules. We subsequently related the surface antigen expression to functional activities of purified neutrophils. From these experiments, we concluded that 7D5-as “early activation” marker--does not necessarily discriminate between primed or resting neutrophils with respect to NADPH oxidase activity.