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Lymphocytosis-promoting factor (LPF) was obtained from culture filtrate of Bordetella pertussis and tested for its effect on the three well-defined cellular hypersensitivity reactions. The development of delayed-type skin reaction with tuberculin PPD and adjuvant disease in the rat was greatly suppressed by intracutaneous injections of a submicrogram dose of LPF. The rejection time of skin homograft in rats and mice was also prolonged by the treatment with LPF. The injection of LPF caused a striking lymphocytosis in the circulation and depletion of lymphocytes in the lymphatic tissues. It is suggested that LPF interacts directly with a selected population of lymphoid cells which participate in the development of cellular hypersensitivity.

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