Intravenous polymethacrylic acid (PMAA) significantly increases the number of lymphocytes in the blood of the rat. The relationship between dose-effect and lymphocytosis is linear. The lethal dose in 30 days of PMAA is 120 mg/kg b.w. and the half-lethal dose 80 mg/kg b.w. The treatment with 40 mg/kg b.w. intravenous PMAA gives no toxic histological changes either in the lymph organs, the liver or the kidneys. Thus, PMAA appears to be, at present, a most suitable agent by which to provoke experimentally, migration of the reserve lymphocytes into the blood.