New ways of understanding milk purity accompanied the introduction of veterinary antibiotics to the dairy farm. Antibiotics—once viewed as substances that could rid milk of bacterial hazards—became understood as food adulterants when residues of the drugs were detected in milk. Although consumers’ concerns about food purity became critical in the postwar era, only in the late 1950s and early 1960s did public health officials and consumers take interest in the potential human health effects of antibiotic residues. Veterinarians and milk processors, not consumers, led the effort to curb indiscriminate use of antibiotics in the 1940s and 1950s. Paying close attention to the context in which farmers chose to use antibiotics and monitored drug residues, this article seeks to explain why farm people adopted veterinary drugs and how they adapted to new regulatory structures and shifting ideas about milk purity.