The authors used cell surface immunofluorescence to investigate the expression of CD45 RA (4KB5)/RO (UCHL1) antigen by acute myeloblastic leukemia (AML) cells from 78 patients. Four types--RA+/RO- (RO < 15%), RA-/RO+ (RA < 15%), mixed (20% RA and 20% RO), and RA-/RO- (RA < 10% and RO < 10%)--were observed. The number of cases with RA+/RO-, RA-/RO+, mixed, and RA-/RO- types in each French-American-British subclass of AML were as follows: M1 (n = 22): 18, 4, 0, 0; M2 (n = 21): 16, 0, 5, 0; M3 (n = 14): 11, 0, 0, 3; M4: 2, 2, 1, 0; and M5: 5, 5, 6, 0, respectively. The M1 RA-/RO+ type, which was always CD7- CD34- HLA-DR-, constituted a rare and distinct M1 subtype because there was no mixed type among the M1 cases. All CD7+ AML cells were the RA+/RO- type and HLA-DR+ except one. The expression of CD45 RO antigen, found in patients with M2, M4, and M5 subtypes of AML, was thought to be associated with the maturity of blasts as monocyte or granulocyte lineage cells. CD45 antigen has tyrosine phosphatase activity in association with nonreceptor-type tyrosine kinases. It was speculated that the functional status and stage of differentiation of the granulocyte/monocyte lineage determine which type of nonreceptor-type tyrosine kinases will operate, which then select the pattern of expression of the CD45 isoform. Thus, the determination of tyrosine kinases associated with CD45 isoforms seems to be important in understanding the AML subsets defined by the pattern of CD45 RA/RO expression.