Acute lymphoblastic leukemia (ALL) patients could be subclassified into 2 groups depending on whether or not their leukemia cells expressed a B[bone marrow-derived]-lymphocyte antigen. The antigen was detected by an indirect immunofluorescence test using rabbit antisera. In the positive group, consisting of 26 of 32 patients, the leukemia cells were of the null cell type, i.e., they did not appear to express currently recognized T[thumus-derived]- and B-cell markers. Absorption studies indicated that the positive null cell group expressed a common antigen which was not expressed on the negative group. Of the 6 negative cases, 5 expressed complement receptors and 3 expressed T-cell markers. The negative group was also characterized by high white cell counts and the presence of a mediastinal mass.