The usefulness of the epinephrine stimulation test in detecting granulocytic hypersplenism is evaluated. In 8 hypersplenic patients, an increment of the granulocyte count of 170% over baseline values was found, compared to 38% (P less than 0.001) in normal subjects and 44% )P less than 0.002) in patients neutropenic from other causes. No overlap of individual test results was found among the hypersplenic and the control groups. Epinephrine test discriminates between neutropenia from hypersplenism and neutropenia from other causes. Furthermore, this test is found to be a valuable adjunct to the hydrocortisone stimulation test in kinetic evaluation of neutropenia from any cause.