The level of stimulation by phytohaemagglutinin (PHA) of mouse lymphocytes of the Balb/c and N strains was dependent on the number of cells and on the lymphoid organ from which the cells were derived. Cells from bone marrow, Peyer’s patches, and the peritoneal exudate showed no increased 3H-thymidine incorporation when incubated with PHA. No correlation between the number of T cells in an organ and the activity of the cells after PHA stimulation was found. Treatment of animals in vivo with cortisone, cyclophosphamide and X-rays caused a reduction in the cell numbers of spleen, thymus and lymph nodes. The activity of the thymus cells of these animals is strongly enhanced in the PHA stimulation test. The activity of spleen and lymph node cells of animals treated with anti-thymocyte serum or X-rays and of lymph node cells of cyclophosphamide-treated animals is reduced. The spleen and lymph node cells of animals treated with anti-thymocyte serum show a high thymidine incorporation without PHA stimulation. In the mixed lymphocyte reaction a total cell number of at least 107 spleen cells was necessary for good stimulation. An optimum was reached after about 4 days. Normal thymus cells were almost inactive in the mixed lymphocyte reaction (MLR). Thymus cells from cortisone-treated animals were very active in the one-way MLR. This result was not obtained with thymus cells of cyclophosphamide-treated animals. Spleen cells from cortisone-treated N mice, but not Balb/c mice showed also an enhanced activity in the ML
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