A rapid assay for tetracyclines in premixes and mixed feeds is described, which uses the extraction and dilution systems of AOAC methods, and a strain of Bacillus stearothermophilus ATCC 12980 selected to grow at 40 degrees C. The incubation period is 4.5 h. The rapid assay yields results similar to those obtained using the AOAC methods. For a 50 g chlortetracycline (CTC)/lb commercial premix, the rapid procedure averaged 109.2% of label vs 104.4% obtained using the AOAC method; for a 20 g CTC/lb premix, the rapid procedure averaged 89.2% vs 89.3% obtained with the AOAC method. In 2 commercial premixes containing 50 g oxytetracycline/lb, the rapid assay averaged 127.1 and 110.5% vs 134.7 and 113.5% obtained using the AOAC method. In feed extracts supplemented with CTC equivalent to 25-200 g/ton, rapid assay recoveries averaged 101.9%; recoveries using the AOAC method averaged 110.6%. For feed extracts supplemented with oxytetracycline at the same levels, recoveries by the rapid assay averaged 95.4%, and by the AOAC method, 106.0%.