A paper disk method based on a procedure described by the International Dairy Federation is presented for the detection of 4 β-lactam antibiotics (penicillin G, ampicillin, cephapirin, and cloxacillin) in whole milk. Bacillus stearothermophilus var. calidolactis, prepared as a stable spore suspension, was used as the test organism. Levels of 0.005 unit penicillin G/ml, 0.005 μg ampicillin or cephapirin/ml, and 0.05 μg cloxacillin/ml were readily detected in whole milk. Results were produced in 3–4 hr. The method offers several advantages, including greater simplicity, sensitivity, and rapidity, over methods now commonly used to detect residual levels of these drugs in milk.