In male dd-mice given two subcutaneous injections of 0.5 mg of hydrocortisone at 35 and 36 days of age, lymphocytes in the bone marrow were studied qualitatively and quantitatively by electron microscopy. In normal 35-day-old mice, lymphocytes comprise 16.4% of nucleated free cells of the marrow. Marrow lymphocytes were, for the most part, of small type which constituted 15.5% of nucleated marrow cells. Small lymphocytes consisted of two types; dark and light small lymphocytes. At 35 days of age, dark small lymphocytes constituted 94.4% of all the marrow small lymphocytes and light small lymphocytes were the remaining 5.6%. After administration of hydrocortisone, lymphocytes in the marrow decreased markedly. The proportion of lymphocytes failed to 6.1% at 1 day, 4.5% at 5 days and 3.5% at 10 days after injection. Thereafter it slightly increased to 5.0% at 20 days after injection. The percentage of marrow lymphocytes at 20 days after injection was almost the same as that of controls at 60 days of age. Of the two types of marrow small lymphocytes dark small lymphocytes decreased rapidly after administration of hydrocortisone. They found 22.4% of the marrow small lymphocytes at 1 day and 10.7% at 5 days after injection. Thereafter, they increased gradually and occupied 58.0% of marrow small lymphocytes at 20 days. On the other hand, light small lymphocytes appeared more resistant to hydrocortisone than dark small lymphocytes.
