new mutant strain of Bordetella bronchiseptica is prepared by applying a mutagen to a strain of B. bronchiseptica to obtain a mutant strain having hereditary markers of at least temperature-sensivity and urease negative (ts-34.u) and subjecting the resulting phase III organism to repeated culturing on a blood agar medium, each time picking up a cell grown with a comparatively small colony, to obtain a phase I organism which has completely capsular antigen and is hemolytic. This strain has excellent immunogenicity and high safety, and is useful for preparation of a live attenuated vaccine for prophylaxis of B. bronchiseptica infection.
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