Summary56 infections episodes in patients with a neoplastic disease were treated by intravenous doxycycline (200 mg per day) ; 37 (66 %) responded favourably to therapy.Minocycline (200 mg per day) (orally) has been effective in the treatment of infections in 35 (77 %) out of 45 patients.In infections caused by sensitive organism, both antibiotics were shown to be an effective therapy ; the rate of clinical failures increased when the causative organism was resistant in vitro ; it is therefore concluded that these antibiotics should not be used in severe infections without testing the sensitivity of the suspected pathogen, chiefly if Gram negative rods are involved, since their sensitivity is less common than for Gram positive cocci.Both antibiotics were well tolerated. A serious complication of treatment with these tetracycline analogues was a high incidence of colonization and superinfection observed in the patients treated.