Therapeutic effect of doxycycline (α-6-deoxyoxytetracycline, DOTC) was compared with that of tetracycline (TC) in experimental acute infection of Staphylococcus aureus in mice. The ED₅₀ of DOTC was half of that of TC in subcutaneous therapy and th e former was one sixth of the latter in oral therapy. No significant differ ence was observed between the effect of both drugs in the infection with Escherichia coli.