A new histochemical technique for morphological studies of mononuclear cells and granulocytes, based on fluorescent staining with methyl green, pyronine Y and stilbene-isothiocyanato disulfonic acid (MPS stain) was described. The method was applied to mononuclear cells isolated from the blood of normal human subjects by Ficoll-Isopaque density centrifugation. Three cell populations were distinguished, mainly on the basis of differences in morphology and cytochemistry, utilizing the MPS stain. One of the cell types had many of the morphological characteristics of the monocyte. This technique, augmented by lysosomal content and endocytosis capacity studies, revealed contamination of the cell suspension with a larger percentage of monocytes than was usually reported in the literature.