For 60 cases of acute lymphoblastic leukemia (ALL) immunological typing was done concurrently by the avidin-biotin-peroxidase method using cytocentrifuged smears and by flow cytofluorometry for the study of surface antigens. The use of a large panel of antibodies detecting differentiation antigens allowed us to sub-classify 57/60 cases as 43 B-lineage ALLs and 14 T-lineage ALLs. The two types of ALL can be accurately distinguished by the expression of the antigens recognized by the antibodies of the clusters of differentiation CD19 (B4) and CD7 (Leu 9). Almost perfect agreement was obtained between the results of the two methods for antigens DR, CD10 (cALLA;J5) and CD7. A number of discordances were observed with other antigens [CD19 (B4), CD20 (B1), CD22 (To15), CD1 (T6), CD2 (T11), CD4 (T4), CD8 (T8), CD3 (T3), T9, T10]. In spite of these discordances, the avidin-biotin-peroxidase method can predict the lineage involved in most ALLs with a high degree of reliability. Nevertheless, for weakly expressed surface antigens (such as B4 and B1) the immunocytological method is less sensitive than flow cytofluorometry and can only approximately determine the stage of differentiation of neoplastic cells. Furthermore, the existence of cases which are at the same time negative with flow cytofluorometry and positive with immunocytology is consistent with the intracytoplasmic expression of certain differentiation antigens. Thus in the course of lymphoid differentiation, intra-cytoplasmic expression of T3, To15 and possibly J5 precedes their expression at the cell surface.