In 83 children with acute lymphoblastic leukaemia (ALL) the immunological phenotype of the lymphoblasts was determined using E rosetting, monoclonal anti-T cell sera, surface immunoglobulin staining and common ALL antiserum. The data were compared with acid alpha-naphthyl acetate esterase (ANAE) and acid phosphatase (AP) cytochemical data. The vast majority of T-ALL cells proved to be ANAE-negative (30/31) and AP-positive (29/31). Null-ALL was always AP-negative (12/12), the ANAE reaction was sometimes positive (4/12). In common ALL, findings ranged from negative to positive for both enzymes. Two cases of B-ALL were ANAE-negative and AP-negative. Enzyme cytochemical determinations gave thus preferential patterns, especially for T-ALL and null-ALL. In common ALL, considerable heterogeneity was found, which may be a reflection of differences in maturation between different cases of common ALL.