1. Conditions for the respiratory adaptation of Saccharomyces cerevisiae were examined, and show that respiratory adaptation of yeast depends on the concentration of glucose and the amount of organisms in the aeration medium.
2. The addition of 100 μg/ml of the toxohormone-like substances from the cancer tissue and RD mutant of yeast showed partially depressing effects on the respiratory adaptation of catalase in yeast, while the toxohormone-like substances from normal yeast showed no effect.
3. Other peptide-containing substances except the toxohormone-like substances did not show any activity to depress the respiratory adaptive production of catalase in yeast, and we confirmed that the activities of the toxohormone-like substances to catalase were specific.

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FL cell cultures infected with the Nakazaki strain of Rickettsia sennetsu were studied. The rickettsiae propagated themselves in the FL cells for a long period of time without interfering with host cell growth. The infected culture has been successively subcultured to establish the persistent infection of rickettsia in vitro. The persistence of the rickettsia in the cultures has been confirmed at each transfer by microscopic examination and by titration in mice.
The rickettsia in this system had retained their infectivity for mice and FL cells. However, the death of mice inoculated with the rickettsia carried in vitro, was remarkably prolonged as the number of transfer incerased.
The study characterized the mutual co-existence of host cells and rickettsiae. The balanced multiplication of them might have enabled the FL cell culture to remain persistently infected. The role of cell-to-cell infection, as well as division of infected cell, was discussed in relation to the establishment of persistent rickettsial infection in vitro.

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1. Type 12, type 14 and unknown type group A streptococci were serially transferred in penicillin (Pc)-, streptomycin (SM)-, tetracycline (TC)-, actinomycin C-, and mitomycin C (MC)-gradient agar.
2. Among resistant mutants obtained, the SM and TC resistant mutants retained β-hemolytic activity on blood agar, while all of 3 MC and 2 Pc resistant mutants lost β-hemolysis and showed α-hemolysis.
3. The cross resistance between Pc and MC resistance was observed.
4. Sugar fermentation of resistant mutants did not differ from that of the parent strains, but mouse virulence decreased.
5. Group and type specific antigenicities of α-hemolytic variants were weakened or lost, but no consistent pattern was apparent.
6. α-Hemolytic variants lost the ability to produce streptolysin S, streptolysin O, and hyaluronidase productions, and their streptokinase activity was lowered.
7. The natural occurrence of antibiotic resistant group A streptococci and the difficulties presented in their isolation from humans is discussed.

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