This unit describes the isolation of monocytes from lymphocytes by adherence, gradient sedimentation on colloidal silica particles, and flow cytometry. Because the first two methods can result in cell activation (induction of gene expression or protein secretion), and the third is technically difficult, a fourth protocol is presented which describes counterflow centrifugal elutriation. This latter procedure can be used to isolate large numbers of purified, nonactivated monocytes.