A total of 15 strains of Mycoplasmawere examined for in vitro sensitivity to 22 commonly used antibiotics and 9 nitrofurans. They were strains of Mycoplasma mycoides var. mycoides, M. mycoides var. capriy M. hyorhinis, M. suipneumoniae (hyopneumoniae), M. granularum, M. cams, M. pulmonis, M. arthritidiSy M. neurolyticum, M. gallisepticum, and M. laidlawii, all of which were isolated from various animals, except for one strain of M. laidlawii which was isolated from sewage. The sensitivity was determined by observing inhibition of growth in the agar and broth dilution systems. Amongall the mycoplasmasexamined, there were no markeddifferences in susceptibility to these drugs, with the exception of erythromycin and oleandomycin. Anti-tumor antibiotics, i. e., actinomycin D and mitomycin C, were the most active of all the agents. Tylosin, bottromycin, spiramycin and tetracycline followed them in activity. Kasugamycin, polymyxin B and colistin were noninhibitory. M.suipneumoniaeå , which is known as the etiological agent of swine enzootic pneumonia (SEP), and other species of respiratory mycoplasmas of swine were compared with regard to minimuminhibitory concentrations (MIC) upon these drugs. The sensitivity of M. suipneumoniae to the drugs used was similar to that of other mycoplasma. Amongthe new nitrofurans tested, drugs with high activity against the mycoplasma were discovered. Manystudies have been made previously on the sensitivity of various mycoplasmas to antimicrobial drugs. Because of the high host specificity of mycoplasmas, strains derived from the same individual host species were chiefly used in such studies.

Establishment of lactobacilli in the digestive tract of germ-free chicks was studied using the organisms isolated from chickens, human infants and sour milk and those received from the American Type Culture Collection. In the case of monocontamination, intestinal lactobacilli such as Lactobacillus acidophilus, L. salivarius and L. It was shown that lactobacilli were predominant in the digestive tract of chickens [11,15], and the lactobacillus flora was composed mainly of Lactobacillus acidophilus, L. salivarius and L. fermenti but rarely included other species [7]. Furthermore, the biotype of L. acidophilus from the chicken and the human showed a considerable difference [7].In the present studies, monocontamination and dicontamination of germ-free chicks with bacteria were attempted to shed light on the problems of why Lactobacillus species such as L. helveticus, L. jugurti, L. brevis, L. plantarum and L. casei are absent from the gut of chickens [7] and why the biotype I of L. acidophilus is rarely found in the gut of chickens [7]. It was expected that the monocontamination would determine whether or not chickens reject the colonization of the above species and L. acidophilus biotype I, and the dicontamination would determine whether or not an antagonistic activity of the indigenous microflora in the gut of chickens result in 1 Present address: