The blasts of 37 adult and 126 childhood cases of acute lymphoblastic leukaemia (ALL) were characterized with a panel of xeno-antisera and rosette tests. The Orthoclone monoclonal antibodies (OK series) were applied as well. Like other investigators, we were able to distinguish 4 major classes of ALL: T-ALL, common-ALL with the subclass pre-B-ALL, null-ALL, and B-ALL. We did not encounter a common-ALL antigen-positive T-ALL subclass. In both adult and childhood ALL, all classes were present, and in about the same frequency as reported by others. In children, common-ALL was the most frequent (66%); in adults, null-ALL (38%). T-ALL was seen both in adults and in children with about the same frequency (27 and 23%, respectively). We found pre-B-ALL only in children. Patients with B-ALL comprised the smallest group in both adult and children (8 and 1.5%, respectively). The application of the OKT antibodies led to recognition of 3 major subclasses of T-ALL: an immature, a common thymocyte and a mature thymocyte subclass. These antibodies were helpful in defining a better classification of null-ALL. With regard to remission induction and prognosis in adult ALL, complete remissions were always obtained in T-ALL, followed by 70% of complete remissions in common-ALL. The worst prognosis was encountered in null-ALL and B-ALL, with 50 and 0% remission, respectively, and a shorter survival in null-ALL of those patients who achieved complete remission. Thus, in high number of cases of null-ALL in adults partly explains the generally much worse prognosis for adult ALL.