A human leukemia-associated antigen (LAA) has been Identified by Immunofluorescence and electrophoretic analyses. LAA was detected on the surfaces of cells from patients with acute lymphocytic leukemia (ALL) as well as on the surfaces of leukemia cells from the established cell lines NALM-1, NALM- 16, MOLT-4, CCRF-CEM, and RPMI 8402. The antigen was not detected on BALM-1 or Raji cells (established B-cell lines), bone marrow cells from ALL patients In remission, or on blood lymphocytes from normal donors. This antigen was most frequently associated with common ALL (cALL); however, cells from 2 of 12 patients with T-cell ALL and 1 patient with B-cell ALL also expressed this antigen. Under reduced conditions, the antigen had an approximate molecular mass of 100, 000 daltons as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiographic analysis and appeared to be the same cALL antigen that has recently been described by others. The probability that LAA Is a normal differentiation antigen was discussed.