Pretreatment peripheral blood and/or bone marrow blasts from 90 adults with acute lymphoblastic leukemia (ALL) were analyzed as part of a prospective treatment protocol study. Specimens were tested by immunofluorescence cytofluorometry for reactivity with the following monoclonal antibodies (MoAbs): BA-1 (B cell antigen); T101, OKT11 (pan-T cell antigens [T]); 3A1 (T cell antigen); MCS-2 (myeloid antigen); J5 common ALL antigen (CALLA); BA4 (Ia antigen [Ia]); BA-2 (lymphohematopoietic antigen). Four major phenotypic groups were identified: B lineage ALL (BA-1+T-) (64%), T lineage ALL (T+BA-1-MCS-2-) (13%), unclassified ALL (BA-1-MCS-2- CALLA-T-) (9%) and myeloid antigen ALL (MCS-2+ CALLA-T-) (7%). An additional group of patients, miscellaneous ALL (7%), was comprised of cases with unusual marker profiles. In B lineage ALL, all cases tested were Ia+MCS-2-, and the vast majority were CALLA+ (84%). In T lineage ALL, 42% expressed CALLA or Ia positivity. In unclassified ALL, the predominant phenotype was Ia+BA-2+. In myeloid antigen ALL, 2 of 4 tested were 3A1+ and all cases evaluated were BA-1-. Patients with myeloid antigen ALL were older (median age, 66 years) than patients in the other groups. The T lineage ALL group had higher leukocyte counts (median WBCs, 183,000/μL) and an increased incidence of anterior mediastinal mass at presentation. All patients received identical induction therapy. In CALLA+ B lineage ALL, 30 of 46 (65%) achieved a complete remission. While the number of patients evaluated was small, 9 of 9 CALLA- B-lineage ALL and only 2 of 6 myeloid antigen ALL cases responded with a complete remission. The data suggest that these MoAbs are useful in the characterization of adult ALL.