The invention provides a method of enumerating the number of cells of a cell type in a cell sample by (a) counting the white blood cells in the cell sample to obtain the white blood cell population of the sample; (b) determining the proportion or percentage of the cells of the cell type in the white blood cell population in the sample; and (c) calculating the number of cells of the cell type in the sample. The cell type may be a lymphocyte sub-set selected from the group comprising CD4+ lymphocytes, CD 45 cells, CD19 cells, CD16 and CD56 positive cells, CD8 cells, CD3 cells or any combination thereof. The method is particularly useful in monitoring the immune status of a patient infected with HIV or other immune deficiency state or disease or condition where CD4+ lymphocytes or CD4+ T cells are monitored or counted.