Dendritic cells (DC) from human peripheral blood, known to adhere transiently and to become non-adherent by 16 hr, can be separated in the low-density interface on hypertonic Metrizamide gradients. Many more low-density cells (5.8% of the mononuclear cells separated on Ficoll) were obtained from the population that was non-adherent after only 90 min. Over 95% of these low-density cells had veiled morphology. A proportion were monocytes by phenotypic and phagocytic properties. One-third of the cells (on average) were DC on the basis of lack of monocyte phenotype and of potency as stimulators in the mixed lymphocyte reaction. Including both the 16 hr and 90 min non-adherent cells, over 2% of the mononuclear cells isolated from human peripheral blood may be DC.