Immunization of BALB/c male mice with human peripheral leukemic blasts effectively reduced the later formation of syngeneic fetal liver, but not bone marrow hematopoietic colonies in the spleen when these mice were lethally irradiated and challenged i.v. Fetal antigen was detected in 6 of 6 lymphocytic leukemic patients and in 4 or 8 myelocytic leukemia patients and was correlated with low cellular levels of sialic acid. A rabbit antiserum ot BALB/c 15-day fetal liver cells labeled only 0 to 2% of normal donor peripheral leukocytes in indirect immunofluorescence but reacted with 10 to 21% of leukemic peripheral blasts. Active disease bone marrow on the same patients gave 7 to 40% fluorescent cells. Two remission bone marrow smaples were negative and 1 had 44% fluorescent cells. Using this antiserum coupled to sepharose, affinity column separation of KCl extracts from mouse and human fetal liver and from chronic lymphocytic leukemia has produced 4 common protein bands (identifiable on polyacrylamide gel electrophoresis).