An analytical method for the determination of tilmicosin in swine feed at 200 to 400 ppm level was evaluated in an interlaboratory study involving five laboratories, including the sponsor. The study evaluated the laboratory precision and accuracy of the method. Each laboratory assayed five replicates of fortified feed at concentrations of 0, 100, 200, 400, and 600 mg/kg. The mean recovery among fortified samples ranged from 81.4 to 98.8 percent with a percent coefficient of variation (%CV) ranging from 0.3 to 4.0 percent. No significant interferences were detected in the control feed samples. A set of blind feed samples was assayed and included five replicates of medicated feed samples prepared at two levels (200 and 400 mg/kg) with either a horizontal or vertical mixer. Five replicates of a control feed were included with the medicated feed samples. The results for the medicated feed samples ranged from 95.8 to 106 percent of label claim with a %CV ranging from 2.1 to 6.7 percent. The results for the control, fortified and medicated feed samples met all the appropriate performance criteria. CVM accepted this method.