The sera from 12 out of 29 half-sibs of the New Zealand White breed of rabbit were inactive, when used as complement, in blood-typing tests of bovine erythrocytes. This defect was found to be due to a deficiency in the activity of the component C6. A dilution of 1:380 of pooled normal rabbit sera and a dilution of 1:520 of pooled normal bovine sera were capable of restoring, to 63%, the haemolytic capacity of the defective rabbit sera. The rate of haemolysis varied from 4% to 100% when the defective sera were restored by sera from different sets of monozygous bovine twins.