1. Blood-stream forms of Trypanosoma brucei ‘O’ strain from infected rats were separated from the blood by differential centrifugation. After washing, the trypanosomes were suspended in modified Ringer-phosphate solution with 0·2m glucose, and normal or immune rabbit serum was added. The oxygen uptake of such trypanosome suspensions was measured at 37° C. for 2 hr. with differential manometers using microflasks of 5 ml. capacity. 2. Immune sera that affected the motility of the trypanosomes also caused a decrease in oxygen uptake. 3. Some trypanosomes appeared to be unaffected by the less active concentrations of immune sera, while other trypanosomes in the same suspension were clumped together or lysed. 4. The most active serum inhibited oxygen uptake less at the highest concentration tested (1/4·5) than at lower concentrations (1/9 and 1/20). 5. This technique was less sensitive than Soltys's method (1957) for demonstrating neutralizing antibodies in the same sera.
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