A dietary factor is thought to be responsible for constant non-genetic neutropenia in Africans. The identity of this factor is unknown. The effect of diet on the differential white cell count in rat is studied. Twenty rats were divided into four dietary groups: (1) control rats on pellets, (2) millet, (3) peanut and (4) a special diet containing high cholesterol and saturated fatty acids from coconut, egg yolk, milk and Danish butter. After 3 months, group 4 rats had significantly higher total white cell counts and percentages of neutrophils in addition to higher serum cholesterol levels and higher weights. In the second experiment, pure cholesterol was injected intraperitoneally to rats while control rats received saline. Neutrophil counts increased 6 h after injection and peaked only in the test rats. It is concluded that low-cholesterol diet decreases neutrophil count.