Necrotic typhlitis, often accompanied with necrotic liver foci, was frequently observed in a conventional rabbit colony during the years 1965-1969. Although different serotypes of Escherichia coli could be cultured from the diseased caeca, many cases occurred from which no Escherichia coli could be isolated. Assuming that the disease might be bacteriological in origin, tetracycline was administered, and mortality decreased. The prevalence of tetracycline-resistant Escherichia coli strains makes application of an alternative therapy desirable.