Technical Abstract: Doramectin concentration in the serum of pastured cattle treated repeatedly at 28 d intervals at two dosage rates was used to predict the probability that cattle fever ticks could successfully feed to repletion during the interval between treatments. At ~270 µg/kg, the doramectin concentration dropped below the baseline level for successful tick feeding (8 ppb) in 7 of the 12 treatments. However, the longest period during which the concentration remained below the baseline level was 15 d, making it impossible for ticks to attain ovipositional status prior to a subsequent treatment. At ~540 µg/kg, the concentration dropped below the baseline level of 8 ppb only once, and the duration during which it remained below the baseline level was only 6 days. Thus, results clearly indicated that no ticks could successfully feed to repletion prior to application of a subsequent treatment. Results also suggested doramectin was metabolized more rapidly during the summer and fall months (July through October), when temperatures were higher, than during the spring and early summer months (late February through mid June), when temperatures were lower. Based on these data, doramectin concentration in serum of treated animals would be a reliable predictor for assessing the probability that ticks could successfully develop to repletion. More importantly, results demonstrated that the U.S. Cattle Fever Tick Eradication Program trial policy of treating cattle with doramectin injections at 25-28 d intervals would produce virtually no risk of allowing viable ticks to reach repletion and sustain the field population.