We studied the incidence of infection in lacerations in a controlled, double-blind, Armitage sequential clinical trial. After standard prescribed preparation of the wound area, and immediately before suture, each laceration was flooded with the 10 ml content of a numbered, amber glass vial. Half of the vials contained a solution of 0.9% NaCl; half contained a 5% solution of sodium benzyl penicillin. No other factors were controlled. A single observer made the determinations of presence or absence of infection, purulent or nonpurulent, early and late after suture. After study of 260 lacerations the study indicated a clear superiority of penicillin over saline in lowering the incidence of infection with a statistical significance of p < 0.00005. In a sample of this size the 95% confidence limits of the magnitude of the superiority of penicillin cover a wide range, but it appears that two out of three or three out of four infections can be averted merely by flooding the wound with penicillin immediately before suture.