To search in children with acute lymphoblastic leukaemia (ALL) for specific pattern of expression of foetal haemoglobin (HbF) and its G gamma/A gamma chain ratio.60 children with ALL were examined: 29 with ALL-L1, and 31 with ALL-L2, and 25 healthy children as control group, which were subdivided in three groups: A) 0-5, B) 6-10 and C) 11-18 years. We performed HbF and HbA2 quantification and Hb electrophoresis. G gamma and A gamma globin chain percentages were obtained with a new method based on the precipitation of the HbF eluate by Singer's method with sulphosalycilic acid, the globin chains were separated in polyacrylamide with Triton X-100 and quantified by densitometry.HbF showed similar levels in both ALL groups by the Betke and Singer's methods; (ALL-L1: 2.2 +/- 1.5%, ALL-L2: 2.0 +/- 1.2%; and ALL-L1: 2.0 +/- 1.2%, ALL-L2: 2.1 +/- 1.5% respectively), but there were statistically significant differences (p < 0.001) when compared with the control group (0.9 +/- 0.4%, and 1.0 +/- 0.6% for Betke and Singer's method). The G gamma/A gamma ratio showed to be different between the ALL-L1 and ALL-L2 (p < 0.001), with higher levels of G gamma in ALL-L1 (51.0%), the ALL-L2 and the control group showed similar G gamma values (37.5% and 42.1% respectively).The factors involved in the increase of HbF are similar for both ALL-L1 and ALL-L2. However there seems to be different factors affecting the expression of G gamma or A gamma.