Human venous blood, anticoagulated with EDTA, was layered onto a discontinuous Percoll gradient, made from solutions of density 1.088, 1.079, and 1.070 gm/ml. After centrifugation at 700g for 15 min at 22 degrees C, the majority of the blood basophils was found in a narrow band at the density 1.070-1.079 interface (Percoll band 2). For 15 normal donors, mean total basophil number recovered from all locations in the gradient was 3.8 +/- 1.2 (SD) X 10(4) basophils per ml of blood applied. Thus, 95% of the values ranged from 1.5 to 6 X 10(4), which compares favorably with the reported range of 1 to 8 X 10(4) basophils per ml for normal subjects. In the basophil-rich Percoll band 2, 2.8 +/- 0.8 X 10(4) basophils were recovered per ml of blood applied. The mean percentage of basophils in Percoll band 2 was 19%, with a range of 5 to 53%. Monocytes and neutrophils were present in very small numbers; the majority of accompanying cells were small lymphocytes.