Antimicrobial resistance is one of the most serious threats to global health, but little progress has been made in reversing its spread. Inappropriate use of antibiotics in humans is a major driver of antimicrobial resistance, and rates are high and growing in lower- and middle-income countries. Antibiotics are thought to be subject to supplier-induced demand, whereby providers prescribe them to patients who do not know they are unnecessary. We conducted a randomized field experiment in 227 private health facilities in Tanzania, with standardized patients presenting uncomplicated upper respiratory tract infection symptoms. Standardized patients were randomly assigned to express knowledge (informed) or not (uninformed) that antibiotics were not required to treat them. There was a very high rate of inappropriate antibiotic prescription, with 86.0 percent of informed standardized patients and 94.8 percent of uninformed standardized patients prescribed an antibiotic, for an adjusted difference of 7.8 percentage points between the groups. This small effect suggests that broader health systems factors are at play and that interventions should be aimed at systems, health facilities, and providers.