Doxycycline is a broad spectrum antibacterial from tetracycline group. It pro duces its antibacterial effect by bindingreversibly to 30S ribosomal subunit of the organism resulting in inhibition ofmicrobial protein synthesis. Doxycycline is active against Gram + ve bacteria & Gram – ve bacteria includ ing E. coli & Pasteurella spp. as well as Mycoplasma spp.