A method for classifying and counting nucleated bone marrow cells comprises the steps of: (1) mixing a sample of bone marrow fluid with an erythrocyte lysing agent to lyse erythrocytes in the sample and render leukocytic cells and erythroid cells in the sample suitable for staining, and staining the sample with a fluorescent dye for producing a difference in intensity of fluorescence between the leukocytic cells and the erythroid cells; (2) introducing the resulting sample to a flow cytometer to detect at least one kind of scattered light and at least one kind of fluorescence; (3) classifying and counting nucleated bone marrow cells, the leukocytic cells and the erythroid cells with use of a difference in the intensity of the fluorescence and the scattered light; (4) calculating the ratio of the nucleated bone marrow cells to the erythroid cells or leukocytic cells from the obtained erythroid cell count or leukocytic cell count and the obtained nucleated bone marrow cell count; and (5) calculating the ratio of the leukocytic cells to the erythroid cells from the erythroid cell count and the leukocytic cell count.