The in vitro antimicrobial activity of the siderophore, deferoxamine, alone and in combination with ascorbic acid, was investigated against 10 clinical isolates of S. aureus employing the broth dilution test and the time-kill method. By the broth dilution test neither activity of deferoxamine and ascorbic acid was demonstrated. In the time-kill study the combination of deferoxamine and ascorbic acid showed a growth inhibiting effect of 3-6 log units at 6 hours. The growth inhibition demonstrated at 6 hours was overcome at 24 hours but could either partially or completely be maintained when repeated addition of the combination of deferoxamine and ascorbic acid or ascorbic acid alone was performed. The antimicrobial effect of deferoxamine could be abolished by adding ferric citrate in ample amounts to saturate deferoxamine with iron.