Treatment of 64 patients having urticaria of unknown aetiology with antibacterial drugs for I to 2 weeks brought relief in 27. Out of the remaining 37 patients, 6 obtained relief on receiving a second different antibacterial drugs. In a control group of 18 patients having w-ticaria of known aeti6logy, none obtained any relief with antibacterial agents. A double blind comparison between tetracycline and a placebo in another group of 18 patients having urticaria of unknown aetiology showed tetracycline to be superior. It seems worthwhile trying a course of antibacterial agents in patients having urticaria of unknown aetiology.