The basophil leukocyte rarely enters into clinical thinking, since it is usually absent from the differential count. However, a determination of the absolute basophil count (ABC) can provide significant information. The present report describes a new technique for the ABC which gives reproducible counts coupled with excellent visualization of the cell morphology. A count of over 50/cu mm is often a sign of allergic sensitization, whereas a count below 20 regularly accompanies allergic reactions. Basophilia is found in association with myxedema, colitis, polycythemia vera, and myeloid leukemia, as well as in the sensitized individual. Conversely, basopenia occurs in hyperthyroidism, pituitary basophilism (Cushing's disease), and stress reactions, and may follow prolonged steroid therapy, as well as the immediate allergic reaction itself.