The monocytes in human peripheral blood, account for, on average, about 8% of the leukocyte population. They tend to be larger (15-20 µm) than lymphocytes (6-20 µm) and they also have a slightly lower density (Figure 1).

 These properties allow some scope for their separation by centrifugation. Boyum [1] introduced a Nycodenz® density barrier (ρ = 1.068 g/ml) for resolving monocytes and lymphocytes from a leukocyte-rich plasma. This is commercially available as Nycoprep™ 1.068. 

It has a slightly raised osmolality (335 mOsm); this enhances the density difference between the monocytes and the osmotically-sensitive lymphocytes, whose density is increased. The method is very effective and the purity of the monocytes is greater than 90% but the monocytes do not form a distinct band; they are concentrated in the upper half of a broad turbid zone within the Nycoprep™ 1.068.