3 cases of adult acute lymphoblastic leukaemia (ALL), the T-cell nature of which was identified only using a panel of monoclonal antibodies (MoAb), are described. All cases were E-rosette negative, surface immunoglobulin (SmIg) negative, common ALL (CALLA) antigen negative, terminal deoxynucleotidyl transferase (TdT) positive, and acid phosphatase positive. The T-cell origin of the blasts was demonstrated by the positivity with RFA-1, a MoAb which detects an antigen of MW 65-69000 present on the membrane of thymocytes and mature T-lymphocytes. In addition, 2 of the 3 cases were positive with OKT6, which recognizes cortical thymocytes. MoAb directed against more mature T lineage cells (OKT3, OKT4, OKT8, OKT11A) were consistently negative (less than or equal to 12%). These findings indicate that the use of a combination of MoAb is important in detecting individual cases of T-ALL, which otherwise might be classified as undifferentiated acute leukaemia or null-ALL. MoAb detecting a T-cell antigenic determinant of MW 65-69000 (e.g. RFA-1, OKT1, Leu1) appear the most specific reagents for T-ALL.