Cell surface markers of 21 cases of acute lymphocytic leukemia (ALL) were studied with various surface markers, especially by using anti-human B lymphocyte serum (ABS), anti-human thymocyte serum (ATS-T) and anti-human peripheral T lymphocyte serum (ALS-T) which were rendered specific for human B lymphocytes, human thymocytes and human peripheral T lymphocytes. The proportion of cell types in ALL was null cell leukemia 38%, B cell leukemia 38% and T cell leukemia 24%, respectively. T-ALL cells were reactive to ATS-T but not to ALS-T, a fact which suggests their thymic origin. It should be noted that these anti-lymphocyte sera detected T or B marker antigens, even when other markers showed negative. Twelve patients with ALL were also investigated from their clinical pictures. Patients with B cell leukemia had severe signs of anemia and a higher grade of hepato-splenomegalies than other types in ALL. Patients with T cell leukemia were in older age levels and had a poorer prognosis.