Abstract: A method for testing blood for the presence of hemoglobin S wherein a blood sample is vigorously admixed with a two-phase test system for the purpose of determining the presence or absence of hemoglobin S after separation of the phases and wherein the two-phase test system involves an aqueous phase containing a lysing agent, a chemical reducing system and a phosphate ion buffer system; and an immiscible phase of mineral oil. The hemoglobin S presence is determinable by the color of a band formed at the interface of the aqueous and immiscible phases and wherein the invention further includes a composition suitable for testing whole blood for hemoglobin S.