Monocytes and lymphocytes in the buffy coat of the peripheral blood of adult mice were qualitatively and cytometrically examined by light and electron microscopy. Clear distinction between lymphocytes and monocytes could not be made in cell size by light microscopy. Two monocyte subgroups could be defined on the basis of the cell diameter: small and large monocytes. Blood lymphocytes consisted of granular lymphocytes and non-granular lymphocytes, and the latter could be divided into two subgroups: those with high N-C (nuclear-cytoplasmic) ratios and low N-C ratios. Non-granular lymphocytes with low N-C ratios and granular lymphocytes were similar in cell size distribution to monocytes, and large monocytes and granular lymphocytes were largely overlapped in the distribution of N-C ratios. The N-C ratio as well as the cell size is not the first criterion for identification of monocytes in mice. However, the occurrence of subsurface vacuoles and the abundance of cytoplasmic granules in monocytes were useful ultrastructural features for the distinction between mononuclear cells in the peripheral blood
