Platelet yield and monoamine oxidase (MAO) activity per platelet in platelets obtained by the method of Corash, which isolates essentially all platelets from whole blood, and by a single differential centrifugation of whole blood, were compared. Platelets recovered by one spin in a swinging bucket rotor appeared to be a homogeneous population containing a reliable representation of the platelet population obtained by the Corash method since the MAO specific activity yielded by the two methods was not significantly different. The effect of the anticoagulant, temperature, and type of rotor on platelet recovery from blood and MAO activity/platelet were also studied. The MAO and plasma amine oxidaze (PAO) activities obtained from blood anticoagulated with ethylenediaminetetracetate (EDTA) were significantly greater than those found when acid-citrate-dextrose was the anticoagulant. Blood centrifuged in a swinging bucket rotor at room temperature yielded 80-90% of total platelets as compared to the 45-65% recovered with a fixed-angle rotor at 0-4 C. The most efficient method of platelet isolation was found to be a 2 1/2-min room-temperature spin at 600 X g with a swinging bucket rotor using EDTA as anticoagulant.