Two methods have been compared through performing differential blood counts: the routine blood count (routine count = RC) and blood counts from films after concentrating white blood cells (cell concentrate = CC). Normal persons were examined and patients with leukocytes less than 1,000/microliter of peripheral blood. The correlation coefficient between RC and CC is higher for more frequent cell types and lower for less frequent cell types. It is concluded that the preparation of CC is simple and reproducible, and that the time-saving CC method for performing differential blood counts from patients with severe leukopenia may be a worthwhile addition to routine laboratory methods.